This page provides a basic tutorial on understanding, creating and
using regular expressions in Perl. It serves as a complement to the
reference page on regular expressions perlre. Regular expressions
are an integral part of the m//, s///, qr// and split
operators and so this tutorial also overlaps with
perlop/``Regex Quote-Like Operators'' and perlfunc/split.
Perl is widely renowned for excellence in text processing, and regular
expressions are one of the big factors behind this fame. Perl regular
expressions display an efficiency and flexibility unknown in most
other computer languages. Mastering even the basics of regular
expressions will allow you to manipulate text with surprising ease.
What is a regular expression? A regular expression is simply a string
that describes a pattern. Patterns are in common use these days;
examples are the patterns typed into a search engine to find web pages
and the patterns used to list files in a directory, e.g., ls *.txt
or dir *.*. In Perl, the patterns described by regular expressions
are used to search strings, extract desired parts of strings, and to
do search and replace operations.
Regular expressions have the undeserved reputation of being abstract
and difficult to understand. Regular expressions are constructed using
simple concepts like conditionals and loops and are no more difficult
to understand than the corresponding if conditionals and while
loops in the Perl language itself. In fact, the main challenge in
learning regular expressions is just getting used to the terse
notation used to express these concepts.
This tutorial flattens the learning curve by discussing regular
expression concepts, along with their notation, one at a time and with
many examples. The first part of the tutorial will progress from the
simplest word searches to the basic regular expression concepts. If
you master the first part, you will have all the tools needed to solve
about 98% of your needs. The second part of the tutorial is for those
comfortable with the basics and hungry for more power tools. It
discusses the more advanced regular expression operators and
introduces the latest cutting edge innovations in 5.6.0.
A note: to save time, 'regular expression' is often abbreviated as
regexp or regex. Regexp is a more natural abbreviation than regex, but
is harder to pronounce. The Perl pod documentation is evenly split on
regexp vs regex; in Perl, there is more than one way to abbreviate it.
We'll use regexp in this tutorial.
NO YOU WON'T, EDITED 12/2001 -- BOBH
The simplest regex is simply a word, or more generally, a string of
characters. A regex consisting of a word matches any string that
contains that word:
"Hello World" =~ /World/; # matches
What is this perl statement all about? "Hello World" is a simple
double quoted string. World is the regular expression and the
// enclosing /World/ tells perl to search a string for a match.
The operator =~ associates the string with the regex match and
produces a true value if the regex matched, or false if the regex
did not match. In our case, World matches the second word in
"Hello World", so the expression is true. Expressions like this
are useful in conditionals:
if ("Hello World" =~ /World/) {
print "It matches\n";
}
else {
print "It doesn't match\n";
}
There are useful variations on this theme. The sense of the match can
be reversed by using !~ operator:
if ("Hello World" !~ /World/) {
print "It doesn't match\n";
}
else {
print "It matches\n";
}
The literal string in the regex can be replaced by a variable:
$greeting = "World";
if ("Hello World" =~ /$greeting/) {
print "It matches\n";
}
else {
print "It doesn't match\n";
}
If you're matching against the special default variable $_, the
$_ =~ part can be omitted:
$_ = "Hello World";
if (/World/) {
print "It matches\n";
}
else {
print "It doesn't match\n";
}
And finally, the // default delimiters for a match can be changed
to arbitrary delimiters by putting an 'm' out front:
"Hello World" =~ m!World!; # matches, delimited by '!'
"Hello World" =~ m{World}; # matches, note the matching '{}'
"/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin',
# '/' becomes an ordinary char
/World/, m!World!, and m{World} all represent the
same thing. When, e.g., "" is used as a delimiter, the forward
slash '/' becomes an ordinary character and can be used in a regex
without trouble.
Let's consider how different regexs would match "Hello World":
"Hello World" =~ /world/; # doesn't match
"Hello World" =~ /o W/; # matches
"Hello World" =~ /oW/; # doesn't match
"Hello World" =~ /World /; # doesn't match
The first regex world doesn't match because regexs are
case-sensitive. The second regex matches because the substring
'o W' occurs in the string "Hello World" . The space
character ' ' is treated like any other character in a regex and is
needed to match in this case. The lack of a space character is the
reason the third regex 'oW' doesn't match. The fourth regex
'World ' doesn't match because there is a space at the end of the
regex, but not at the end of the string. The lesson here is that
regexs must match a part of the string exactly in order for the
statement to be true.
If a regex matches in more than one place in the string, perl will
always match at the earliest possible point in the string:
"Hello World" =~ /o/; # matches 'o' in 'Hello'
"That hat is red" =~ /hat/; # matches 'hat' in 'That'
With respect to character matching, there are a few more points you
need to know about. First of all, not all characters can be used 'as
is' in a match. Some characters, called metacharacters, are reserved
for use in regex notation. The metacharacters are
{}[]()^$.|*+?\
The significance of each of these will be explained
in the rest of the tutorial, but for now, it is important only to know
that a metacharacter can be matched by putting a backslash before it:
"2+2=4" =~ /2+2/; # doesn't match, + is a metacharacter
"2+2=4" =~ /2\+2/; # matches, \+ is treated like an ordinary +
"The interval is [0,1)." =~ /[0,1)./ # is a syntax error!
"The interval is [0,1)." =~ /\[0,1\)\./ # matches
"/usr/bin/perl" =~ /\/usr\/local\/bin\/perl/; # matches
In the last regex, the forward slash '/' is also backslashed,
because it is used to delimit the regex. This can lead to LTS
(leaning toothpick syndrome), however, and it is often more readable
to change delimiters.
The backslash character '\' is a metacharacter itself and needs to
be backslashed:
'C:\WIN32' =~ /C:\\WIN/; # matches
In addition to the metacharacters, there are some ASCII characters
which don't have printable character equivalents and are instead
represented by escape sequences. Common examples are \t for a
tab, \n for a newline, \r for a carriage return and \a for a
bell. If your string is better thought of as a sequence of arbitrary
bytes, the octal escape sequence, e.g., \033, or hexadecimal escape
sequence, e.g., \x1B may be a more natural representation for your
bytes. Here are some examples of escapes:
"1000\t2000" =~ m(0\t2) # matches
"1000\n2000" =~ /0\n20/ # matches
"1000\t2000" =~ /\000\t2/ # doesn't match, "0" ne "\000"
"cat" =~ /\143\x61\x74/ # matches, but a weird way to spell cat
If you've been around Perl a while, all this talk of escape sequences
may seem familiar. Similar escape sequences are used in double-quoted
strings and in fact the regexs in Perl are mostly treated as
double-quoted strings. This means that variables can be used in
regexs as well. Just like double-quoted strings, the values of the
variables in the regex will be substituted in before the regex is
evaluated for matching purposes. So we have:
$foo = 'house';
'housecat' =~ /$foo/; # matches
'cathouse' =~ /cat$foo/; # matches
'housecat' =~ /${foo}cat/; # matches
So far, so good. With the knowledge above you can already perform
searches with just about any literal string regex you can dream up.
Here is a very simple emulation of the Unix grep program:
% cat > simple_grep
#!/usr/bin/perl
$regex = shift;
while (<>) {
print if /$regex/;
}
^D
% chmod +x simple_grep
% simple_grep abba /usr/dict/words
Babbage
cabbage
cabbages
sabbath
Sabbathize
Sabbathizes
sabbatical
scabbard
scabbards
This program is easy to understand. #!/usr/bin/perl is the standard
way to invoke a perl program from the shell.
$regex = shift; saves the first command line argument as the
regex to be used, leaving the rest of the command line arguments to
be treated as files. while (<>) loops over all the lines in
all the files. For each line, print if /$regex/; prints the
line if the regex matches the line. In this line, both print and
/$regex/ use the default variable $_ implicitly.
With all of the regexs above, if the regex matched anywhere in the
string, it was considered a match. Sometimes, however, we'd like to
specify where in the string the regex should try to match. To do
this, we would use the anchor metacharacters ^ and $. The
anchor ^ means match at the beginning of the string and the anchor
$ means match at the end of the string, or before a newline at the
end of the string. Here is how they are used:
"housekeeper" =~ /keeper/; # matches
"housekeeper" =~ /^keeper/; # doesn't match
"housekeeper" =~ /keeper$/; # matches
"housekeeper\n" =~ /keeper$/; # matches
The second regex doesn't match because ^ constrains keeper to
match only at the beginning of the string, but "housekeeper" has
keeper starting in the middle. The third regex does match, since the
$ constrains keeper to match only at the end of the string.
When both ^ and $ are used at the same time, the regex has to
match both the beginning and the end of the string, i.e., the regex
matches the whole string. Consider
"keeper" =~ /^keep$/; # doesn't match
"keeper" =~ /^keeper$/; # matches
"" =~ /^$/; # ^$ matches an empty string
The first regex doesn't match because the string has more to it than
keep. Since the second regex is exactly the string, it
matches. Using both ^ and $ in a regex forces the complete
string to match, so it gives you complete control over which strings
match and which don't. Suppose you are looking for a fellow named
bert, off in a string by himself:
"dogbert" =~ /bert/; # matches, but not what you want
"dilbert" =~ /^bert/; # doesn't match, but ..
"bertram" =~ /^bert/; # matches, so still not good enough
"bertram" =~ /^bert$/; # doesn't match, good
"dilbert" =~ /^bert$/; # doesn't match, good
"bert" =~ /^bert$/; # matches, perfect
Of course, in the case of a literal string, one could just as easily
use the string equivalence $string eq 'bert' and it would be
more efficient. The ^...$ regex really becomes useful when we
add in the more powerful regex tools below.
Although one can already do quite a lot with the literal string
regexs above, we've only scratched the surface of regular expression
technology. In this and subsequent sections we will introduce regex
concepts (and associated metacharacter notations) that will allow a
regex to not just represent a single character sequence, but a whole
class of them.
One such concept is that of a character class. A character class
allows a set of possible characters, rather than just a single
character, to match at a particular point in a regex. Character
classes are denoted by brackets [...], with the set of characters
to be possibly matched inside. Here are some examples:
/cat/; # matches 'cat'
/[bcr]at/; # matches 'bat, 'cat', or 'rat'
/item[0123456789]/; # matches 'item0' or ... or 'item9'
"abc" =~ /[cab]/; # matches 'a'
In the last statement, even though 'c' is the first character in
the class, 'a' matches because the first character position in the
string is the earliest point at which the regex can match.
/[yY][eE][sS]/; # match 'yes' in a case-insensitive way
# 'yes', 'Yes', 'YES', etc.
This regex displays a common task: perform a a case-insensitive
match. Perl provides away of avoiding all those brackets by simply
appending an 'i' to the end of the match. Then /[yY][eE][sS]/;
can be rewritten as /yes/i;. The 'i' stands for
case-insensitive and is an example of a modifier of the matching
operation. We will meet other modifiers later in the tutorial.
We saw in the section above that there were ordinary characters, which
represented themselves, and special characters, which needed a
backslash \ to represent themselves. The same is true in a
character class, but the sets of ordinary and special characters
inside a character class are different than those outside a character
class. The special characters for a character class are -]\^$. ]
is special because it denotes the end of a character class. $ is
special because it denotes a scalar variable. \ is special because
it is used in escape sequences, just like above. Here is how the
special characters ]$\ are handled:
/[\]c]def/; # matches ']def' or 'cdef'
$x = 'bcr';
/[$x]at/; # matches 'bat', 'cat', or 'rat'
/[\$x]at/; # matches '$at' or 'xat'
/[\\$x]at/; # matches '\at', 'bat, 'cat', or 'rat'
The last two are a little tricky. in [\$x], the backslash protects
the dollar sign, so the character class has two members $ and x.
In [\\$x], the backslash is protected, so $x is treated as a
variable and substituted in double quote fashion.
The special character '-' acts as a range operator within character
classes, so that a contiguous set of characters can be written as a
range. With ranges, the unwieldy [0123456789] and [abc...xyz]
become the svelte [0-9] and [a-z]. Some examples are
/item[0-9]/; # matches 'item0' or ... or 'item9'
/[0-9bx-z]aa/; # matches '0aa', ..., '9aa',
# 'baa', 'xaa', 'yaa', or 'zaa'
/[0-9a-fA-F]/; # matches a hexadecimal digit
/[0-9a-zA-Z_]/; # matches an alphanumeric character,
# like those in a perl variable name
If '-' is the first or last character in a character class, it is
treated as an ordinary character; [-ab], [ab-] and [a\-b] are
all equivalent.
The special character ^ in the first position of a character class
denotes a negated character class, which matches any character but
those in the brackets. Both [...] and [^...] must match a
character, or the match fails. Then
/[^a]at/; # doesn't match 'aat' or 'at', but matches
# all other 'bat', 'cat, '0at', '%at', etc.
/[^0-9]/; # matches a non-numeric character
/[a^]at/; # matches 'aat' or '^at'; here '^' is ordinary
Now, even [0-9] can be a bother the write multiple times, so in the
interest of saving keystrokes and making regexs more readable, Perl
has several abbreviations for common character classes:
The \d\s\w\D\S\W abbreviations can be used both inside and outside
of character classes. Here are some in use:
/\d\d:\d\d:\d\d/; # matches a hh:mm:ss time format
/[\d\s]/; # matches any digit or whitespace character
/\w\W\w/; # matches a word char, followed by a
# non-word char, followed by a word char
/..rt/; # matches any two chars, followed by 'rt'
/end\./; # matches 'end.'
/end[.]/; # same thing, matches 'end.'
Because a period is a metacharacter, it needs to be escaped to match
as an ordinary period. Because, for example, \d and \w are sets
of characters, it is incorrect to think of [^\d\w] as [\D\W]; in
fact [^\d\w] is the same as [^\w], which is the same as
[\W]. Think DeMorgan's laws.
An anchor useful in basic regexs is the word anchor
\b. This matches a boundary between a word character and a non-word
character \w\W or \W\w:
$x = "Housecat catenates house and cat";
$x =~ /cat/; # matches cat in 'housecat'
$x =~ /\bcat/; # matches cat in 'catenates'
$x =~ /cat\b/; # matches cat in 'housecat'
$x =~ /\bcat\b/; # matches 'cat' at end of string
Note in the last example, the end of the string is considered a word
boundary.
You might wonder why '.' matches everything but "\n" - why not
every character? The reason is that often one is matching against
lines and would like to ignore the newline characters. For instance,
while the string "\n" represents one line, we would like to think
of as empty. Then
"" =~ /^$/; # matches
"\n" =~ /^$/; # matches, "\n" is ignored
"" =~ /./; # doesn't match; it needs a char
"" =~ /^.$/; # doesn't match; it needs a char
"\n" =~ /^.$/; # doesn't match; it needs a char other than "\n"
"a" =~ /^.$/; # matches
"a\n" =~ /^.$/; # matches, ignores the "\n"
This behavior is convenient, because we usually want to ignore
newlines when we count and match characters in a line. Sometimes,
however, we want to keep track of newlines. We might even want ^
and $ to anchor at the beginning and end of lines within the
string, rather than just the beginning and end of the string. Perl
allows us to choose between ignoring and paying attention to newlines
by using the //s and //m modifiers. //s and //m stand for
single line and multi-line and they determine whether a string is to
be treated as one continuous string, or as a set of lines. The two
modifiers affect two aspects of how the regex is interpreted: 1) how
the '.' character class is defined, and 2) where the anchors ^
and $ are able to match. Here are the four possible combinations:
Here are examples of //s and //m in action:
$x = "There once was a girl\nWho programmed in Perl\n";
$x =~ /^Who/; # doesn't match, "Who" not at start of string
$x =~ /^Who/s; # doesn't match, "Who" not at start of string
$x =~ /^Who/m; # matches, "Who" at start of second line
$x =~ /^Who/sm; # matches, "Who" at start of second line
$x =~ /girl.Who/; # doesn't match, "." doesn't match "\n"
$x =~ /girl.Who/s; # matches, "." matches "\n"
$x =~ /girl.Who/m; # doesn't match, "." doesn't match "\n"
$x =~ /girl.Who/sm; # matches, "." matches "\n"
Most of the time, the default behavior is what is want, but //s and
//m are occasionally very useful. If //m is being used, the start
of the string can still be matched with \A and the end of string
can still be matched with the anchors \Z (matches both the end and
the newline before, like $), and \z (matches only the end):
$x =~ /^Who/m; # matches, "Who" at start of second line
$x =~ /\AWho/m; # doesn't match, "Who" is not at start of string
$x =~ /girl$/m; # matches, "girl" at end of first line
$x =~ /girl\Z/m; # doesn't match, "girl" is not at end of string
$x =~ /Perl\Z/m; # matches, "Perl" is at newline before end
$x =~ /Perl\z/m; # doesn't match, "Perl" is not at end of string
We now know how to create choices among classes of characters in a
regex. What about choices among words or character strings? Such
choices are described in the next section.
Sometimes we would like to our regex to be able to match different
possible words or character strings. This is accomplished by using
the alternation metacharacter |. To match dog or cat, we
form the regex dog|cat. As before, perl will try to match the
regex at the earliest possible point in the string. At each
character position, perl will first try to match the first
alternative, dog. If dog doesn't match, perl will then try the
next alternative, cat. If cat doesn't match either, then the
match fails and perl moves to the next position in the string. Some
examples:
"cats and dogs" =~ /cat|dog|bird/; # matches "cat"
"cats and dogs" =~ /dog|cat|bird/; # matches "cat"
Even though dog is the first alternative in the second regex,
cat is able to match earlier in the string.
"cats" =~ /c|ca|cat|cats/; # matches "c"
"cats" =~ /cats|cat|ca|c/; # matches "cats"
Here, all the alternatives match at the first string position, so the
first alternative is the one that matches. If some of the
alternatives are truncations of the others, put the longest ones first
to give them a chance to match.
"cab" =~ /a|b|c/ # matches "c"
# /a|b|c/ == /[abc]/
The last example points out that character classes are like
alternations of characters. At a given character position, the first
alternative that allows the regex match to succeed wil be the one
that matches.
Alternation allows a regex to choose among alternatives, but by
itself it unsatisfying. The reason is that each alternative is a whole
regex, but sometime we want alternatives for just part of a
regex. For instance, suppose we want to search for housecats or
housekeepers. The regex housecat|housekeeper fits the bill, but is
inefficient because we had to type house twice. It would be nice to
have parts of the regex be constant, like house, and and some
parts have alternatives, like cat|keeper.
The grouping metacharacters () solve this problem. Grouping
allows parts of a regex to be treated as a single unit. Parts of a
regex are grouped by enclosing them in parentheses. Thus we could solve
the housecat|housekeeper by forming the regex as
house(cat|keeper). The regex house(cat|keeper) means match
house followed by either cat or keeper. Some more examples
are
/(a|b)b/; # matches 'ab' or 'bb'
/(ac|b)b/; # matches 'acb' or 'bb'
/(^a|b)c/; # matches 'ac' at start of string or 'bc' anywhere
/(a|[bc])d/; # matches 'ad', 'bd', or 'cd'
/house(cat|)/; # matches either 'housecat' or 'house'
/house(cat(s|)|)/; # matches either 'housecats' or 'housecat' or
# 'house'. Note groups can be nested.
/(19|20|)\d\d/; # match years 19xx, 20xx, or the Y2K problem, xx
"20" =~ /(19|20|)\d\d/; # matches the null alternative '()\d\d',
# because '20\d\d' can't match
Alternations behave the same way in groups as out of them: at a given
string position, the leftmost alternative that allows the regex to
match is taken. So in the last example at tth first string position,
"20" matches the second alternative, but there is nothing left over
to match the next two digits \d\d. So perl moves on to the next
alternative, which is the null alternative and that works, since
"20" is two digits.
The process of trying one alternative, seeing if it matches, and
moving on to the next alternative if it doesn't, is called
backtracking. The term 'backtracking' comes from the idea that
matching a regex is like a walk in the woods. Successfully matching
a regex is like arriving at a destination. There are many possible
trailheads, one for each string position, and each one is tried in
order, left to right. From each trailhead there may be many paths,
some of which get you there, and some which are dead ends. When you
walk along a trail and hit a dead end, you have to backtrack along the
trail to an earlier point to try another trail. If you hit your
destination, you stop immediately and forget about trying all the
other trails. You are persistent, and only if you have tried all the
trails from all the trailheads and not arrived at your destination, do
you declare failure. To be concrete, here is a step-by-step analysis
of what perl does when it tries to match the regex
"abcde" =~ /(abd|abc)(df|d|de)/;
- Start with the first letter in the string 'a'.
- Try the first alternative in the first group 'abd'.
- Match 'a' followed by 'b'. So far so good.
- 'd' in the regex doesn't match 'c' in the string - a dead
end. So backtrack two characters and pick the second alternative in
the first group 'abc'.
- Match 'a' followed by 'b' followed by 'c'. We are on a roll
and have satisfied the first group. Set $1 to 'abc'.
- Move on to the second group and pick the first alternative
'df'.
- Match the 'd'.
- 'f' in the regex doesn't match 'e' in the string, so a dead
end. Backtrack one character and pick the second alternative in the
second group 'd'.
- 'd' matches. The second grouping is satisfied, so set $2 to
'd'.
- We are at the end of the regex, so we are done! We have
matched 'abcd' out of the string ``abcde''.
There are a couple of things to note about this analysis. First, the
third alternative in the second group 'de' also allows a match, but we
stopped before we got to it - at a given character position, leftmost
wins. Second, we were able to get a match at the first character
position of the string 'a'. If there were no matches at the first
position, perl would move to the second character position 'b' and
attempt the match all over again. Only when all possible paths at all
possible character positions have been exhausted does perl give give
up and declare $string =~ /(abd|abc)(df|d|de)/; to be false.
Even with all this work, regex matching happens remarkably fast. To
speed things up, during compilation stage, perl compiles the regex
into a compact sequence of opcodes that can often fit inside a
processor cache. When the code is executed, these opcodes can then run
at full throttle and search very quickly.
The grouping metacharacters () also serve another completely
different function: they allow the extraction of the parts of a string
that matched. This is very useful to find out what matched and for
text processing in general. For each grouping, the part that matched
inside goes into the special variables $1, $2, etc. They can be
used just as ordinary variables:
# extract hours, minutes, seconds
$time =~ /(\d\d):(\d\d):(\d\d)/; # match hh:mm:ss format
$hours = $1;
$minutes = $2;
$seconds = $3;
Now, we know that in scalar context,
$time =~ /(\d\d):(\d\d):(\d\d)/ returns a true or false
value. In list context, however, it returns the list of matched values
($1,$2,$3). So we could write the code more compactly as
# extract hours, minutes, seconds
($hours, $minutes, $second) = ($time =~ /(\d\d):(\d\d):(\d\d)/);
If the groupings in a regex are nested, $1 gets the group with the
leftmost opening parenthesis, $2 the next opening parenthesis,
etc. For example, here is a complex regex and the matching variables
indicated below it:
/(ab(cd|ef)((gi)|j))/;
1 2 34
so that if the regex matched, e.g., $2 would contain 'cd' or 'ef'.
For convenience, perl sets $+ to the highest numbered $1, $2,
... that got assigned.
Closely associated with the matching variables $1, $2, ... are
the backreferences \1, \2, ... . Backreferences are simply
matching variables that can be used inside a regex. This is a
really nice feature - what matches later in a regex can depend on
what matched earlier in the regex. Suppose we wanted to look
for doubled words in text, like 'the the'. The following regex finds
all 3-letter doubles with a space in between:
/(\w\w\w)\s\1/;
The grouping assigns a value to \1, so that the same 3 letter sequence
is used for both parts. Here are some words with repeated parts:
% simple_grep '^(\w\w\w\w|\w\w\w|\w\w|\w)\1$' /usr/dict/words
beriberi
booboo
coco
mama
murmur
papa
The regex has a single grouping which considers 4-letter
combinations, then 3-letter combinations, etc. and uses \1 to look for
a repeat. Although $1 and \1 represent the same thing, care should be
taken to use matched variables $1, $2, ... only outside a regex
and backreferences \1, \2, ... only inside a regex; not doing
so may lead to surprising and/or undefined results.
In addition to what was matched, Perl 5.6.0 also provides the
positions of what was matched with the @- and @+
arrays. $-[0] is the position of the start of the entire match and
$+[0] is the position of the end. Similarly, $-[n] is the
position of the start of the $n match and $+[n] is the position
of the end. If $n is undefined, so are $-[n] and $+[n]. Then
this code
$x = "Mmm...donut, thought Homer";
$x =~ /^(Mmm|Yech)\.\.\.(donut|peas)/; # matches
foreach $expr (1..$#-) {
print "Match $expr: '${$expr}' at position ($-[$expr],$+[$expr])\n";
}
prints
Match 1: 'Mmm' at position (0,3)
Match 2: 'donut' at position (6,11)
Even if there are no groupings in a regex, it is still possible to
find out what exactly matched in a string. If you use them, perl
will set $` to the part of the string before the match, will set $&
to the part of the string that matched, and will set $' to the part
of the string after the match. An example:
$x = "the cat caught the mouse";
$x =~ /cat/; # $` = 'the ', $& = 'cat', $' = ' caught the mouse'
$x =~ /the/; # $` = '', $& = 'the', $' = ' cat caught the mouse'
In the second match, $` = '' because the regex matched at the
first character position in the string and stopped, it never saw the
second 'the'. It is important to note that using $` and $'
slows down regex matching quite a bit, and $& slows it down to a
lesser extent, because if they are used in one regex in a program,
they are generated for <all> regexs in the program. So if raw
performance is a goal of your application, they should be avoided.
If you need them, use @- and @+ instead:
$` is the same as substr( $x, 0, $-[0] )
$& is the same as substr( $x, $-[0], $+[0]-$-[0] )
$' is the same as substr( $x, $+[0] )
The examples in the previous section display an annoying weakness. We
were only matching 3-letter words, or syllables of 4 letters or
less. We'd like to be able to match words or syllables of any length,
without writing out tedious alternatives like
\w\w\w\w|\w\w\w|\w\w|\w.
This is exactly the problem the quantifier metacharacters ?,
*, +, and {} were created for. They allow us to determine the
number of repeats of a portion of a regex we consider to be a
match. Quantifiers are put immediately after the character, character
class, or grouping that we want to specify. They have the following
meanings:
Here are some examples:
/[a-z]+\s+\d*/; # match a lowercase word, at least some space, and
# any number of digits
/(\w+)\s+\1/; # match doubled words of arbitrary length
/y(es)?/i; # matches 'y', 'Y', or a case-insensitive 'yes'
$year =~ /\d{2,4}/; # make sure year is at least 2 but not more
# than 4 digits
$year =~ /\d{4}|\d{2}/; # better match; throw out 3 digit dates
$year =~ /\d{2}(\d{2})?/; # same thing written differently. However,
# this produces $1 and the other does not.
% simple_grep '^(\w+)\1$' /usr/dict/words # isn't this easier?
beriberi
booboo
coco
mama
murmur
papa
For all of these quantifiers, perl will try to match as much of the
string as possible, while still allowing the regex to succeed. Thus
with /a?.../, perl will first try to match the regex with the a
present; if that fails, perl will try to match the regex without the
a present. For the quantifier *, we get the following:
$x = "the cat in the hat";
$x =~ /^(.*)(cat)(.*)$/; # matches,
# $1 = 'the '
# $2 = 'cat'
# $3 = ' in the hat'
Which is what we might expect, the match finds the only cat in the
string and locks onto it. Consider, however, this regex:
$x =~ /^(.*)(at)(.*)$/; # matches,
# $1 = 'the cat in the h'
# $2 = 'at'
# $3 = '' (0 matches)
One might initially guess that perl would find the at in cat and
stop there, but that wouldn't give the longest possible string to the
first quantifier .*. Instead, the first quantifier .* grabs as
much of the string as possible while still having the regex match. In
this example, that means having the at sequence with the final at
in the string. The other important principle illustrated here is that
when there are two or more elements in a regex, the leftmost
quantifier, if there is one, gets to grab as much the string as
possible, leaving the rest of the regex to fight over scraps. Thus in
our example, the first quantifier .* grabs most of the string, while
the second quantifier .* gets the empty string. Quantifiers that
grab as much of the string as possible are called maximal match or
greedy quantifiers.
When a regex can match a string in several different ways, we can use
the principles above to predict which way the regex will match:
- Principle 0: Taken as a whole, any regex will be matched at the
earliest possible position in the string.
- Principle 1: In an alternation a|b|c..., the leftmost alternative
that allows a match for the whole regex will be the one used.
- Principle 2: The maximal matching quantifiers ?, *, + and
{n,m} will in general match as much of the string as possible while
still allowing the whole regex to match.
- Principle 3: If there are two or more elements in a regex, the
leftmost greedy quantifier, if any, will match as much of the string
as possible while still allowing the whole regex to match. The next
leftmost greedy quantifier, if any, will try to match as much of the
string remaining available to it as possible, while still allowing the
whole regex to match. And so on, until all the regex elements are
satisfied.
As we have seen above, Principle 0 overrides the others - the regex
will be matched as early as possible, with the other principles
determining how the regex matches at that earliest character
position.
Here is an example of these principles in action:
$x = "The programming republic of Perl";
$x =~ /^(.+)(e|r)(.*)$/; # matches,
# $1 = 'The programming republic of Pe'
# $2 = 'r'
# $3 = 'l'
This regex matches at the earliest string position, 'T'. One
might think that e, being leftmost in the alternation, would be
matched, but r produces the longest string in the first quantifier.
$x =~ /(m{1,2})(.*)$/; # matches,
# $1 = 'mm'
# $2 = 'ing republic of Perl'
Here, The earliest possible match is at the first 'm' in
programming. m{1,2} is the first quantifier, so it gets to match
a maximal mm.
$x =~ /.*(m{1,2})(.*)$/; # matches,
# $1 = 'm'
# $2 = 'ing republic of Perl'
Here, the regex matches at the start of the string. The first
quantifier .* grabs as much as possible, leaving just a single
'm' for the second quantifier m{1,2}.
$x =~ /(.?)(m{1,2})(.*)$/; # matches,
# $1 = 'a'
# $2 = 'mm'
# $3 = 'ing republic of Perl'
Here, .? eats its maximal one character at the earliest possible
position in the string, 'a' in programming, leaving m{1,2}
the opportunity to match both m's. Finally,
"aXXXb" =~ /(X*)/; # matches with $1 = ''
because it can match zero copies of 'X' at the beginning of the
string. If you definitely want to match at least one 'X', use
X+, not X*.
Sometimes greed is not good. At times, we would like quantifiers to
match a minimal piece of string, rather than a maximal piece. For
this purpose, Larry Wall created the minimal match or
non-greedy quantifiers ??,*?, +?, and {}?. These are
the usual quantifiers with a ? appended to them. They have the
following meanings:
- a?? = match 'a' 0 or 1 times. Try 0 first, then 1.
- a*? = match 'a' 0 or more times, i.e., any number of times,
but as few times as possible
- a+? = match 'a' 1 or more times, i.e., at least once, but
as few times as possible
- a{n,m}? = match at least n times, not more than m
times, as few times as possible
- a{n,}? = match at least n times, but as few times as
possible
- a{n}? = match exactly n times. Because we match exactly
n times, a{n}? is equivalent to a{n} and is just there for
notational consistency.
Let's look at the example above, but with minimal quantifiers:
$x = "The programming republic of Perl";
$x =~ /^(.+?)(e|r)(.*)$/; # matches,
# $1 = 'Th'
# $2 = 'e'
# $3 = ' programming republic of Perl'
The minimal string that will allow both the start of the string ^
and the alternation to match is Th, with the alternation e|r
matching e. The second quantifier .* is free to gobble up the
rest of the string.
$x =~ /(m{1,2}?)(.*?)$/; # matches,
# $1 = 'm'
# $2 = 'ming republic of Perl'
The first string position that this regex can match is at the first
'm' in programming. At this position, the minimal m{1,2}?
matches just one 'm'. Although the second quantifier .*? would
prefer to match no characters, it is constrained by the end-of-string
anchor $ to match the rest of the string.
$x =~ /(.*?)(m{1,2}?)(.*)$/; # matches,
# $1 = 'The progra'
# $2 = 'm'
# $3 = 'ming republic of Perl'
In this regex, you might expect the first minimal quantifier .*?
to match the empty string, because it is not constrained by a ^
anchor to match the beginning of the word. Principle 0 applies here,
however. Because it is possible for the whole regex to match at the
start of the string, it will match at the start of the string. Thus
the first quantifier has to match everything up to the first m. The
second minimal quantifier matches just one m and the third
quantifier matches the rest of the string.
$x =~ /(.??)(m{1,2})(.*)$/; # matches,
# $1 = 'a'
# $2 = 'mm'
# $3 = 'ing republic of Perl'
Just as in the previous regex, the first quantifier .?? can match
earliest at position 'a', so it does. The second quantifier is
greedy, so it matches mm, and the third matches the rest of the
string.
We can modify principle 3 above to take into account non-greedy
quantifiers:
Just like alternation, quantifiers are also susceptible to
backtracking. Here is a step-by-step analysis of the example
$x = "the cat in the hat";
$x =~ /^(.*)(at)(.*)$/; # matches,
# $1 = 'the cat in the h'
# $2 = 'at'
# $3 = '' (0 matches)
- Start with the first letter in the string 't'.
- The first quantifier '.*' starts out by matching the whole
string 'the cat in the hat'.
- 'a' in the regex element 'at' doesn't match the end of the
string. Backtrack one character.
- 'a' in the regex element 'at' still doesn't match the last
letter of the string 't', so backtrack one more character.
- Now we can match the 'a' and the 't'.
- Move on to the third element '.*'. Since we are at the end of
the string and '.*' can match 0 times, assign it the empty string.
- We are done!
Most of the time, all this moving forward and backtracking happens
quickly and searching is fast. There are some pathological regexs,
however, whose execution time exponentially grows with the size of the
string. A typical structure that blows up in your face is of the form
/(a|b+)*/;
The problem is the nested indeterminate quantifiers. There are many
different ways of partitioning a string of length n between the +
and *: one repetition with b+ of length n, two repetitions with
the first b+ length k and the second with length n-k, m repetitions
whose bits add up to length n, etc. In fact there are an exponential
number of ways to partition a string as a function of length. A
regex may get lucky and match early in the process, but if there is
no match, perl will try every possibility before giving up. So be
careful with nested *'s, {n,m}'s, and +'s. The book
Mastering regular expressions by Jeffrey Friedl gives a wonderful
discussion of this and other efficiency issues.
At this point, we have all the basic regex concepts covered, so let's
give a more involved example of a regular expression. We will build a
regex that matches numbers.
The first task in building a regex is to decide what we want to match
and what we want to exclude. In our case, we want to match both
integers and floating point numbers and we want to reject any string
that isn't a number.
The next task is to break the problem down into smaller problems that
are easily converted into a regex.
The simplest case is integers. These consist of a sequence of digits,
with an optional sign in front. The digits we can represent with
\d+ and the sign can be matched with [+-]. Thus the integer
regex is
/[+-]?\d+/; # matches integers
A floating point number potentially has a sign, an integral part, a
decimal point, a fractional part, and an exponent. One or more of these
parts is optional, so we need to check out the different
possibilities. Floating point numbers which are in proper form include
123., 0.345, .34, -1e6, and 25.4E-72. As with integers, the sign out
front is completely optional and can be matched by [+-]?. We can
see that if there is no exponent, floating point numbers must have a
decimal point, otherwise they are integers. We might be tempted to
model these with \d*\.\d*, but this would also match just a single
decimal point, which is not a number. So the three cases of floating
point number sans exponent are
/[+-]?\d+\./; # 1., 321., etc.
/[+-]?\.\d+/; # .1, .234, etc.
/[+-]?\d+\.\d+/; # 1.0, 30.56, etc.
These can be combined into a single regex with a three-way alternation:
/[+-]?(\d+\.\d+|\d+\.|\.\d+)/; # floating point, no exponent
In this alternation, it is important to put '\d+\.\d+' before
'\d+\.'. If '\d+\.' were first, the regex would happily match that
and ignore the fractional part of the number.
Now consider floating point numbers with exponents. The key
observation here is that both integers and numbers with decimal
points are allowed in front of an exponent. Then exponents, like the
overall sign, are independent of whether we are matching numbers with
or without decimal points, and can be 'decoupled' from the
mantissa. The overall form of the regex now becomes clear:
/^(optional sign)(integer | f.p. mantissa)(optional exponent)$/;
The exponent is an e or E, followed by an integer. So the
exponent regex is
/[eE][+-]?\d+/; # exponent
Putting all the parts together, we get a regex that matches numbers:
/^[+-]?(\d+\.\d+|\d+\.|\.\d+|\d+)([eE][+-]?\d+)?$/; # Ta da!
Long regexs like this may impress your friends, but can be hard to
decipher. In complex situations like this, the //x modifier for a
match is invaluable. It allows one to put nearly arbitrary whitespace
and comments into a regex without affecting their meaning. Using it,
we can rewrite our 'extended' regex in the more pleasing form
/^
[+-]? # first, match an optional sign
( # then match integers or f.p. mantissas:
\d+\.\d+ # mantissa of the form a.b
|\d+\. # mantissa of the form a.
|\.\d+ # mantissa of the form .b
|\d+ # integer of the form a
)
([eE][+-]?\d+)? # finally, optionally match an exponent
$/x;
If whitespace is mostly irrelevant, how does one include space
characters in an extended regex? The answer is to backslash it
'\ ' or put it in a character class [ ] . The same thing
goes for pound signs, use \# or [#]. For instance, Perl allows
a space between the sign and the mantissa/integer, and we could add
this to our regex as follows:
/^
[+-]?\ * # first, match an optional sign *and space*
( # then match integers or f.p. mantissas:
\d+\.\d+ # mantissa of the form a.b
|\d+\. # mantissa of the form a.
|\.\d+ # mantissa of the form .b
|\d+ # integer of the form a
)
([eE][+-]?\d+)? # finally, optionally match an exponent
$/x;
In this form, it is easier to see a way to simplify the
alternation. Alternatives 1, 2, and 4 all start with \d+, so it
could be factored out:
/^
[+-]?\ * # first, match an optional sign
( # then match integers or f.p. mantissas:
\d+ # start out with a ...
(
\.\d* # mantissa of the form a.b or a.
)? # ? takes care of integers of the form a
|\.\d+ # mantissa of the form .b
)
([eE][+-]?\d+)? # finally, optionally match an exponent
$/x;
or written in the compact form,
/^[+-]?\ *(\d+(\.\d*)?|\.\d+)([eE][+-]?\d+)?$/;
This is our final regex. To recap, we built a regex by
These are also the typical steps involved in writing a computer
program. This makes perfect sense, because regular expressions are
essentially programs written a little computer language that specifies
patterns.
The last topic of Part 1 briefly covers how regexs are used in Perl
programs. Where do they fit into Perl syntax?
We have already introduced the matching operator in its default
/regex/ and arbitrary delimiter m!regex! forms. We have used
the binding operator =~ and its negation !~ to test for string
matches. Associated with the matching operator, we have discussed the
single line //s, multi-line //m, case-insensitive //i and
extended //x modifiers.
There are a few more things you might want to know about matching
operators. First, we pointed out earlier that variables in regexs are
substituted before the regex is evaluated:
$pattern = 'Seuss';
while (<>) {
print if /$pattern/;
}
This will print any lines containing the word Seuss. It is not as
efficient as it could be, however, because perl has to re-evaluate
$pattern each time through the loop. If $pattern won't be
changing over the lifetime of the script, we can add the //o
modifier, which directs perl to only perform variable substitutions
once:
#!/usr/bin/perl
# Improved simple_grep
$regex = shift;
while (<>) {
print if /$regex/o; # a good deal faster
}
If you change $pattern after the first substitution happens, perl
will ignore it. If you don't want any substitutions at all, use the
special delimiter m'':
$pattern = 'Seuss';
while (<>) {
print if m'$pattern'; # matches '$pattern', not 'Seuss'
}
m'' acts like single quotes on a regex; all other m delimiters
act like double quotes. If the regex evaluates to the empty string,
the regex in the last successful match is used instead. So we have
"dog" =~ /d/; # 'd' matches
"dogbert =~ //; # this matches the 'd' regex used before
The final two modifiers //g and //c concern multiple matches.
The modifier //g stands for global matching and allows the the
matching operator to match within a string as many times as possible.
In scalar context, successive invocations against a string will have
`//g jump from match to match, keeping track of position in the
string as it goes along. You can get or set the position with the
pos() function.
The use of //g is shown in the following example. Suppose we have
a string that consists of words separated by spaces. If we know how
many words there are in advance, we could extract the words using
groupings:
$x = "cat dog house"; # 3 words
$x =~ /^\s*(\w+)\s+(\w+)\s+(\w+)\s*$/; # matches,
# $1 = 'cat'
# $2 = 'dog'
# $3 = 'house'
But what if we had an indeterminate number of words? This is the sort
of task //g was made for. To extract all words, form the simple
regex (\w+) and loop over all matches with /(\w+)/g:
while ($x =~ /(\w+)/g) {
print "Word is $1, ends at position ", pos $x, "\n";
}
prints
Word is cat, ends at position 3
Word is dog, ends at position 7
Word is house, ends at position 13
A failed match or changing the target string resets the position. If
you don't want the position reset after failure to match, add the
//c, as in /regex/gc. The current position in the string is
associated with the string, not the regex. This means that different
strings have different positions and their respective positions can be
set or read independently.
In list context, //g returns a list of matched groupings, or if
there are no groupings, a list of matches to the whole regex. So if
we wanted just the words, we could use
@words = ($x =~ /(\w+)/g); # matches,
# $word[0] = 'cat'
# $word[1] = 'dog'
# $word[2] = 'house'
Closely associated with the //g modifier is the \G anchor. The
\G anchor matches at the point where the previous //g match left
off. \G allows us to easily do context-sensitive matching:
$metric = 1; # use metric units
...
$x = <FILE>; # read in measurement
$x =~ /^([+-]?\d+)\s*/g; # get magnitude
$weight = $1;
if ($metric) { # error checking
print "Units error!" unless $x =~ /\Gkg\./g;
}
else {
print "Units error!" unless $x =~ /\Glbs\./g;
}
$x =~ /\G\s+(widget|sprocket)/g; # continue processing
The combination of //g and \G allows us to process the string a
bit at a time and use arbitrary Perl logic to decide what to do next.
\G is also invaluable in processing fixed length records with
regexs. Suppose we have a snippet of coding region DNA, encoded as
base pair letters ATCGTTGAAT... and we want to find all the stop
codons TGA. In a coding region, codons are 3-letter sequences, so
we can think of the DNA snippet as a sequence of 3-letter records. The
naive regex
# expanded, this is "ATC GTT GAA TGC AAA TGA CAT GAC"
$dna = "ATCGTTGAATGCAAATGACATGAC";
$dna =~ /TGA/;
doesn't work; it may match an TGA, but there is no guarantee that
the match is aligned with codon boundaries, e.g., the substring
GTT GAA gives a match. A better solution is
while ($dna =~ /(\w\w\w)*?TGA/g) { # note the minimal *?
print "Got a TGA stop codon at position ", pos $dna, "\n";
}
which prints
Got a TGA stop codon at position 18
Got a TGA stop codon at position 23
Position 18 is good, but position 23 is bogus. What happened?
The answer is that our regex works well until we get past the last
real match. Then the regex will fail to match a synchronized TGA
and start stepping ahead one character position at a time, not what we
want. The solution is to use \G to anchor the match to the codon
alignment:
while ($dna =~ /\G(\w\w\w)*?TGA/g) {
print "Got a TGA stop codon at position ", pos $dna, "\n";
}
This prints
Got a TGA stop codon at position 18
which is the correct answer. This example illustrates that it is
important not only to match what is desired, but to reject what is not
desired.
search and replace
Regular expressions also play a big role in search and replace
operations in Perl. Search and replace is accomplished with the
s/// operator. The general form is
s/regex/replacement/modifiers, with everything we know about
regexs and modifiers applying in this case as well. The
replacement is a Perl double quoted string that replaces in the
string whatever is matched with the regex. The operator =~ is
also used here to associate a string with s///. If matching
against $_, the $_ =~ can be dropped. If there is a match,
s/// returns the number of substitutions made, otherwise it returns
false. Here are a few examples:
$x = "Time to feed the cat!";
$x =~ s/cat/hacker/; # $x contains "Time to feed the hacker!"
if ($x =~ s/^(Time.*hacker)!$/$1 now!/) {
$more_insistent = 1;
}
$y = "'quoted words'";
$y =~ s/^'(.*)'$/$1/; # strip single quotes,
# $y contains "quoted words"
In the last example, the whole string was matched, but only the part
inside the single quotes was grouped. With the s/// operator, the
matched variables $1, $2, etc. are immediately available for use
in the replacement expression, so we use $1 to replace the quoted
string with just what was quoted. With the global modifier, s///g
will search and replace all occurrences of the regex in the string:
$x = "I batted 4 for 4";
$x =~ s/4/four/; # doesn't do it all:
# $x contains "I batted four for 4"
$x = "I batted 4 for 4";
$x =~ s/4/four/g; # does it all:
# $x contains "I batted four for four"
If you prefer 'regex' over 'regex' in this tutorial, you could use
the following program to replace it:
% cat > simple_replace
#!/usr/bin/perl
$regex = shift;
$replacement = shift;
while (<>) {
s/$regex/$replacement/go;
print;
}
^D
% simple_replace regex regex perlretut.pod
In simple_replace we used the s///g modifier to replace all
occurrences of the regex on each line and the s///o modifier to
compile the regex only once. As with simple_grep, both the
print and the s/$regex/$replacement/go use $_ implicitly.
A modifier available specifically to search and replace is the
s///e evaluation modifier. s///e wraps an eval{...} around
the replacement string and the evaluated result is substituted for the
matched substring. s///e is useful if you need to do a bit of
computation in the process of replacing text. This example counts
character frequencies in a line:
$x = "Bill the cat";
$x =~ s/(.)/$chars{$1}++;$1/eg; # final $1 replaces char with itself
print "frequency of '$_' is $chars{$_}\n"
foreach (sort {$chars{$b} <=> $chars{$a}} keys %chars);
This prints
frequency of ' ' is 2
frequency of 't' is 2
frequency of 'l' is 2
frequency of 'B' is 1
frequency of 'c' is 1
frequency of 'e' is 1
frequency of 'h' is 1
frequency of 'i' is 1
frequency of 'a' is 1
As with the match m// operator, s/// can use other delimiters,
such as s!!! and s{}{}, and even s{}//. If single quotes are
used s''', then the regex and replacement are treated as single
quoted strings and there are no substitutions. s/// in list context
returns the same thing as in scalar context, i.e., the number of
matches.
The split operator
The split function can also optionally use a matching operator
m// to split a string. split /regex/, string, limit splits
string into a list of substrings and returns that list. The regex
is used to match the character sequence that the string is split
with respect to. The limit, if present, constrains splitting into
no more than limit number of strings. For example, to split a
string into words, use
$x = "Calvin and Hobbes";
@words = split /\s+/, $x; # $word[0] = 'Calvin'
# $word[1] = 'and'
# $word[2] = 'Hobbes'
If the empty regex // is used, the regex always matches and
the string is split into individual characters. If the regex has
groupings, then list produced contains the matched substrings from the
groupings as well. For instance,
$x = "/usr/bin/perl";
@dirs = split m!/!, $x; # $dirs[0] = ''
# $dirs[1] = 'usr'
# $dirs[2] = 'bin'
# $dirs[3] = 'perl'
@parts = split m!(/)!, $x; # $parts[0] = ''
# $parts[1] = '/'
# $parts[2] = 'usr'
# $parts[3] = '/'
# $parts[4] = 'bin'
# $parts[5] = '/'
# $parts[6] = 'perl'
Since the first character of $x matched the regex, split prepended
an empty initial element to the list.
If you have read this far, congratulations! You now have all the basic
tools needed to use regular expressions to solve a wide range of text
processing problems. If this is your first time through the tutorial,
why not stop here and play around with regexs a while... Part 2
concerns the more esoteric aspects of regular expressions and those
concepts certainly aren't needed right at the start.
OK, you know the basics of regexs and you want to know more. If
matching regular expressions is analogous to a walk in the woods, then
the tools discussed in Part 1 are analogous to topo maps and a
compass, basic tools we use all the time. Most of the tools in part 2
are are analogous to flare guns and satellite phones. They aren't used
too often on a hike, but when we are stuck, they can be invaluable.
What follows are the more advanced, less used, or sometimes esoteric
capabilities of perl regexs. In Part 2, we will assume you are
comfortable with the basics and concentrate on the new features.
There are a number of escape sequences and character classes that we
haven't covered yet.
There are several escape sequences that convert characters or strings
between upper and lower case. \l and \u convert the next
character to lower or upper case, respectively:
$x = "perl";
$string =~ /\u$x/; # matches 'Perl' in $string
$x = "M(rs?|s)\\."; # note the double backslash
$string =~ /\l$x/; # matches 'mr.', 'mrs.', and 'ms.',
\L and \U converts a whole substring, delimited by \L or
\U and \E, to lower or upper case:
$x = "This word is in lower case:\L SHOUT\E";
$x =~ /shout/; # matches
$x = "I STILL KEYPUNCH CARDS FOR MY 360"
$x =~ /\Ukeypunch/; # matches punch card string
If there is no \E, case is converted until the end of the
string. The regexs \L\u$word or \u\L$word convert the first
character of $word to uppercase and the rest of the characters to
lowercase.
Control characters can be escaped with \c, so that a control-Z
character would be matched with \cZ. The escape sequence
\Q...\E quotes, or protects most non-alphabetic characters. For
instance,
$x = "\QThat !^*&%~& cat!";
$x =~ /\Q!^*&%~&\E/; # check for rough language
It does not protect $ or @, so that variables can still be
substituted.
With the advent of 5.6.0, perl regexs can handle more than just the
standard ASCII character set. Perl now supports Unicode, a standard
for encoding the character sets from many of the world's written
languages. Unicode does this by allowing characters to be more than
one byte wide. Perl uses the UTF-8 encoding, in which ASCII characters
are still encoded as one byte, but characters greater than chr(127)
may be stored as two or more bytes.
What does this mean for regexs? Well, regex users don't need to know
much about perl's internal representation of strings. But they do need
to know 1) how to represent Unicode characters in a regex and 2) when
a matching operation will treat the string to be searched as a
sequence of bytes (the old way) or as a sequence of Unicode characters
(the new way). The answer to 1) is that Unicode characters greater
than chr(127) may be represented using the \x{hex} notation,
with hex a hexadecimal integer:
use utf8; # We will be doing Unicode processing
/\x{263a}/; # match a Unicode smiley face :)
Unicode characters in the range of 128-255 use two hexadecimal digits
with braces: \x{ab}. Note that this is different than \xab,
which is just a hexadecimal byte with no Unicode
significance.
Figuring out the hexadecimal sequence of a Unicode character you want
or deciphering someone else's hexadecimal Unicode regex is about as
much fun as programming in machine code. So another way to specify
Unicode characters is to use the named character escape
sequence \N{name}. name is a name for the Unicode character, as
specified in the Unicode standard. For instance, if we wanted to
represent or match the astrological sign for the planet Mercury, we
could use
use utf8; # We will be doing Unicode processing
use charnames ":full"; # use named chars with Unicode full names
$x = "abc\N{MERCURY}def";
$x =~ /\N{MERCURY}/; # matches
One can also use short names or restrict names to a certain alphabet:
use utf8; # We will be doing Unicode processing
use charnames ':full';
print "\N{GREEK SMALL LETTER SIGMA} is called sigma.\n";
use charnames ":short";
print "\N{greek:Sigma} is an upper-case sigma.\n";
use charnames qw(greek);
print "\N{sigma} is Greek sigma\n";
A list of full names is found in the file Names.txt in the
lib/perl5/5.6.0/unicode directory.
The answer to requirement 2), as of 5.6.0, is that if a regex
contains Unicode characters, the string is searched as a sequence of
Unicode characters. Otherwise, the string is searched as a sequence of
bytes. If the string is being searched as a sequence of Unicode
characters, but matching a single byte is required, we can use the \C
escape sequence. \C is a character class akin to . except that
it matches any byte 0-255. So
use utf8; # We will be doing Unicode processing
use charnames ":full"; # use named chars with Unicode full names
$x = "a";
$x =~ /\C/; # matches 'a', eats one byte
$x = "";
$x =~ /\C/; # doesn't match, no bytes to match
$x = "\N{MERCURY}"; # two-byte Unicode character
$x =~ /\C/; # matches, but dangerous!
The last regex matches, but is dangerous because the string
character position is no longer synchronized to the string byte
position. This generates the warning 'Malformed UTF-8
character'. \C is best used for matching the binary data in strings
with binary data intermixed with Unicode characters.
Let us now discuss the rest of the character classes. Just as with
Unicode characters, there are named Unicode character classes
represented by the \p{name} escape sequence. Closely associated is
the \P{name} character class, which is the negation of the
\p{name} class. For example, to match lower and uppercase
characters,
use utf8; # We will be doing Unicode processing
use charnames ":full"; # use named chars with Unicode full names
$x = "BOB";
$x =~ /^\p{IsUpper}/; # matches, uppercase char class
$x =~ /^\P{IsUpper}/; # doesn't match, char class sans uppercase
$x =~ /^\p{IsLower}/; # doesn't match, lowercase char class
$x =~ /^\P{IsLower}/; # matches, char class sans lowercase
If a name is just one letter, the braces can be dropped. For
instance, \pM is the character class of Unicode 'marks'. Here is
the association between some Perl named classes and the traditional
Unicode classes:
Perl class name Unicode class name
IsAlpha Lu, Ll, or Lo
IsAlnum Lu, Ll, Lo, or Nd
IsASCII $code le 127
IsCntrl C
IsDigit Nd
IsGraph [^C] and $code ne "0020"
IsLower Ll
IsPrint [^C]
IsPunct P
IsSpace Z, or ($code lt "0020" and chr(hex $code) is a \s)
IsUpper Lu
IsWord Lu, Ll, Lo, Nd or $code eq "005F"
IsXDigit $code =~ /^00(3[0-9]|[46][1-6])$/
For a full list of Perl class names, consult the mktables.PL program
in the lib/perl5/5.6.0/unicode directory.
\X is an abbreviation for a character class sequence that includes
the Unicode 'combining character sequences'. A 'combining character
sequence' is a base character followed by any number of combining
characters. An example of a combining character is an accent. Using
the Unicode full names, e.g., A + COMBINING RING is a combining
character sequence with base character A and combining character
COMBINING RING , which translates in Danish to A with the circle
atop it, as in the word Angstrom. \X is equivalent to \PM\pM*},
i.e., a non-mark followed by one or more marks.
As if all those classes weren't enough, Perl also defines POSIX style
character classes. These have the form [:name:], with name the
name of the POSIX class. The POSIX classes are alpha, alnum, ascii,
cntrl, digit, graph, lower, print, punct, space, upper, word, and
xdigit. If utf8 is being used, then these classes are defined the
same as their corresponding perl Unicode classes: [:upper:] is the
same as \p{IsUpper}, etc. The POSIX character classes, however,
don't require using utf8. The [:digit:], [:word:], and
[:space:] correspond to the familiar \d, \w, and \s
character classes. To negate a POSIX class, put a ^ in front of the
name, so that, e.g., [:^digit:] corresponds to \D and under
utf8, \P{IsDigit}. The Unicode and POSIX character classes can
be used just like \d, both inside and outside of character classes:
/\s+[abc[:digit:]xyz]\s*/; # match a,b,c,x,y,z, or a digit
/^=item\s[:digit:]/; # match '=item',
# followed by a space and a digit
use utf8;
use charnames ":full";
/\s+[abc\p{IsDigit}xyz]\s+/; # match a,b,c,x,y,z, or a digit
/^=item\s\p{IsDigit}/; # match '=item',
# followed by a space and a digit
Whew! That is all the rest of the characters and character classes.
In Part 1 we discussed the //o modifier, which compiles a regex
just once. This suggests that a compiled regex is some data structure
that can be stored once and used again and again. The regex quote
qr// does exactly that: qr/string/ compiles the string as a
regex and transforms the result into a form that can be assigned to a
variable:
$reg = qr/foo+bar?/; # reg contains a compiled regex
Then $reg can be used as a regex:
$x = "fooooba";
$x =~ $reg; # matches, just like /foo+bar?/
$x =~ /$reg/; # same thing, alternate form
$reg can also be interpolated into a larger regex:
$x =~ /(abc)?$reg/; # still matches
As with the matching operator, the regex quote can use different
delimiters, e.g., qr!!, qr{} and qr~~. The single quote
delimiters qr'' prevent any interpolation from taking place.
Pre-compiled regexs are useful for creating dynamic matches that
don't need to be recompiled each time they are encountered. Using
pre-compiled regexs, simple_grep program can be expanded into a
program that matches multiple patterns:
% cat > multi_grep
#!/usr/bin/perl
# multi_grep - match any of <number> regexs
# usage: multi_grep <number> regex1 regex2 ... file1 file2 ...
$number = shift;
$regex[$_] = shift foreach (0..$number-1);
@compiled = map qr/$_/, @regex;
while ($line = <>) {
foreach $pattern (@compiled) {
if ($line =~ /$pattern/) {
print $line;
last; # we matched, so move onto the next line
}
}
}
^D
% multi_grep 2 last for multi_grep
$regex[$_] = shift foreach (0..$number-1);
foreach $pattern (@compiled) {
last;
Storing pre-compiled regexs in an array @compiled allows us to
simply loop through the regexs without any recompilation, thus gaining
flexibility without sacrificing speed.
Starting with this section, we will be discussing Perl's set of
extended patterns. These are extensions to the traditional regular
expression syntax that provide powerful new tools for pattern
matching. We have already seen extensions in the form of the minimal
matching constructs ??, *?, +?, {n,m}?, and {n,}?. The
rest of the extensions below have the form (?char...), where the
char is a character that determines the type of extension.
The first extension is an embedded comment (?#text). This embeds a
comment into the regular expression without affecting its meaning. The
comment should not have any closing parentheses in the text. An
example is
/(?# Match an integer:)[+-]?\d+/;
This style of commenting has been largely superseded by the raw,
freeform commenting that is allowed with the //x modifier.
The modifiers //i, //m, //s, and //x can also embedded in
a regex using (?i), (?m), (?s), and (?x). For instance,
/(?i)yes/; # match 'yes' case insensitively
/yes/i; # same thing
/(?x)( # freeform version of an integer regex
[+-]? # match an optional sign
\d+ # match a sequence of digits
)
/x;
Embedded modifiers can have two important advantages over the usual
modifiers. Embedded modifiers allow a custom set of modifiers to
each regex pattern. This is great for matching an array of regexs
that must have different modifiers:
$pattern[0] = '(?i)doctor';
$pattern[1] = 'Johnson';
...
while (<>) {
foreach $patt (@pattern) {
print if /$patt/;
}
}
The second advantage is that embedded modifiers only affect the regex
inside the group the embedded modifier is contained in. So grouping
can be used to localize the modifier's effects:
/Answer: ((?i)yes)/; # matches 'Answer: yes', 'Answer: YES', etc.
Embedded modifiers can also turn off any modifiers already present
by using, e.g., (?-i). Modifiers can also be combined into
a single expression, e.g., (?s-i) turns on single line mode and
turns off case insensitivity.
We noted in Part 1 that groupings () had two distinct functions: 1)
group regex elements together as a single unit, and 2) extract, or
capture, substrings that matched the regex in the
grouping. Non-capturing groupings, denoted by (?:regex), allow the
regex to be treated as a single unit, but don't extract substrings or
set matching variables $1, etc. Both capturing and non-capturing
groupings are allowed to co-exist in the same regex. Because there is
no extraction, non-capturing groupings are faster than capturing
groupings. Non-capturing groupings are also handy for choosing exactly
which parts of a regex are to be extracted to matching variables:
# match a number, $1-$4 are set, but we only want $1
/([+-]?\ *(\d+(\.\d*)?|\.\d+)([eE][+-]?\d+)?)/;
# match a number faster , only $1 is set
/([+-]?\ *(?:\d+(?:\.\d*)?|\.\d+)(?:[eE][+-]?\d+)?)/;
# match a number, get $1 = whole number, $2 = exponent
/([+-]?\ *(?:\d+(?:\.\d*)?|\.\d+)(?:[eE]([+-]?\d+))?)/;
Non-capturing groupings are also useful for removing nuisance
elements gathered from a split operation:
$x = '12a34b5';
@num = split /(a|b)/, $x; # @num = ('12','a','34','b','5')
@num = split /(?:a|b)/, $x; # @num = ('12','34','5')
Non-capturing groupings may also have embedded modifiers:
(?i-m:regex) is a non-capturing grouping that matches regex
case insensitively and turns off multi-line mode.
This section concerns the lookahead and lookbehind assertions. First,
a little background.
In Perl regular expressions, most regex elements 'eat up' a certain
amount of string when they match. For instance, the regex element
[abc}] eats up one character of the string when it matches, in the
sense that perl moves to the next character position in the string
after the match. There are some elements, however, that don't eat up
characters (advance the character position) if they match. The examples
we have seen so far are the anchors. The anchor ^ matches the
beginning of the line, but doesn't eat any characters. Similarly, the
word boundary anchor \b matches, e.g., if the character to the left
is a word character and the character to the right is a non-word
character, but it doesn't eat up any characters itself. Anchors are
examples of 'zero-width assertions'. Zero-width, because they consume
no characters, and assertions, because they test some property of the
string. In the context of our walk in the woods analogy to regex
matching, most regex elements move us along a trail, but anchors have
us stop a moment and check our surroundings. If the local environment
checks out, we can proceed forward. But if the local environment
doesn't satisfy us, we must backtrack.
Checking the environment entails either looking ahead on the trail,
looking behind, or both. ^ looks behind, to see that there are no
characters before. $ looks ahead, to see that there are no
characters after. \b looks both ahead and behind, to see if the
characters on either side differ in their 'word'-ness.
The lookahead and lookbehind assertions are generalizations of the
anchor concept. Lookahead and lookbehind are zero-width assertions
that let us specify which characters we want to test for. The
lookahead assertion is denoted by (?=regex) and the lookbehind
assertion is denoted by (?<=fixed-regex). Some examples are
$x = "I catch the housecat 'Tom-cat' with catnip";
$x =~ /cat(?=\s+)/; # matches 'cat' in 'housecat'
@catwords = ($x =~ /(?<=\s)cat\w+/g); # matches,
# $catwords[0] = 'catch'
# $catwords[1] = 'catnip'
$x =~ /\bcat\b/; # matches 'cat' in 'Tom-cat'
$x =~ /(?<=\s)cat(?=\s)/; # doesn't match; no isolated 'cat' in
# middle of $x
Note that the parentheses in (?=regex) and (?<=regex) are
non-capturing, since these are zero-width assertions. Thus in the
second regex, the substrings captured are those of the whole regex
itself. Lookahead (?=regex) can match arbitrary regexs, but
lookbehind (?<=fixed-regex) only works for regexs of fixed
width, i.e., a fixed number of characters long. Thus
(?<=(ab|bc)) is fine, but (?<=(ab)*) is not. The
negated versions of the lookahead and lookbehind assertions are
denoted by (?!regex) and (?<!fixed-regex) respectively.
They evaluate true if the regexs do not match:
$x = "foobar";
$x =~ /foo(?!bar)/; # doesn't match, 'bar' follows 'foo'
$x =~ /foo(?!baz)/; # matches, 'baz' doesn't follow 'foo'
$x =~ /(?<!\s)foo/; # matches, there is no \s before 'foo'
The last few extended patterns in this tutorial are experimental as of
5.6.0. Play with them, use them in some code, but don't rely on them
just yet for production code.
Independent subexpressions are regular expressions, in the
context of a larger regular expression, that function independently of
the larger regular expression. That is, they consume as much or as
little of the string as they wish without regard for the ability of
the larger regex to match. Independent subexpressions are represented
by (?>regex). We can illustrate their behavior by first
considering an ordinary regex:
$x = "ab";
$x =~ /a*ab/; # matches
This obviously matches, but in the process of matching, the
subexpression a* first grabbed the a. Doing so, however,
wouldn't allow the whole regex to match, so after backtracking, a*
eventually gave back the a and matched the empty string. Here, what
a* matched was dependent on what the rest of the regex matched.
Contrast that with an independent subexpression:
$x =~ /(?>a*)ab/; # doesn't match!
The independent subexpression (?>a*) doesn't care about the rest
of the regex, so it sees an a and grabs it. Then the rest of the
regex ab cannot match. Because (?>a*) is independent, there
is no backtracking and and the independent subexpression does not give
up its a. Thus the match of the regex as a whole fails. A similar
behavior occurs with completely independent regexs:
$x = "ab";
$x =~ /a*/g; # matches, eats an 'a'
$x =~ /\Gab/g; # doesn't match, no 'a' available
Here //g and \G create a 'tag team' handoff of the string from
one regex to the other. Regexs with an independent subexpression are
much like this, with a handoff of the string to the independent
subexpression, and a handoff of the string back to the enclosing
regex.
The ability of an independent subexpression to prevent backtracking
can be quite useful. Suppose we want to match a non-empty string
enclosed in parentheses up to two levels deep. Then the following
regex matches:
$x = "abc(de(fg)h"; # unbalanced parentheses
$x =~ /\( ( [^()]+ | \([^()]*\) )+ \)/x;
The regex matches an open parenthesis, one or more copies of an
alternation, and a close parenthesis. The alternation is two-way, with
the first alternative [^()]+ matching a substring with no
parentheses and the second alternative \([^()]*\) matching a
substring delimited by parentheses. The problem with this regex is
that it is pathological: it has nested indeterminate quantifiers
of the form (a+|b)+. We discussed in Part 1 how nested quantifiers
like this could take an exponentially long time to execute if there
was no match possible. To prevent the exponential blowup, we need to
prevent useless backtracking at some point. This can be done by
enclosing the inner quantifier as an independent subexpression:
$x =~ /\( ( (?>[^()]+) | \([^()]*\) )+ \)/x;
Here, (?>[^()]+) breaks the degeneracy of string partitioning
by gobbling up as much of the string as possible and keeping it. Then
match failures fail much more quickly.
A conditional expression is a form of if-then-else statement
that allows one to choose which patterns are to be matched, based on
some condition. There are two types of conditional expression:
(?(condition)yes-regex) and
(?(condition)yes-regex|no-regex). (?(condition)yes-regex) is
like an 'if () {}' statement in Perl. If the condition is true,
the yes-regex will be matched. If the condition is false, the
yes-regex will be skipped and perl will move onto the next regex
element. The second form is like an 'if () {} else {}' statement
in Perl. If the condition is true, the yes-regex will be
matched, otherwise the no-regex will be matched.
The condition can have two forms. The first form is simply an
integer in parentheses (integer). It is true if the corresponding
backreference \integer matched earlier in the regex. The second
form is a bare zero width assertion (?...), either a
lookahead, a lookbehind, or a code assertion (discussed in the next
section).
The integer form of the condition allows us to choose, with more
flexibility, what to match based on what matched earlier in the
regex. This searches for words of the form "$x$x" or
"$x$y$y$x":
% simple_grep '^(\w+)(\w+)?(?(2)\2\1|\1)$' /usr/dict/words
beriberi
coco
couscous
deed
...
toot
toto
tutu
The lookbehind condition allows, along with backreferences,
an earlier part of the match to influence a later part of the
match. For instance,
/[ATGC]+(?(?<=AA)G|C)$/;
matches a DNA sequence such that it either ends in AAG, or some
other base pair combination and C. Note that the form is
(?(?<=AA)G|C) and not (?((?<=AA))G|C); for the
lookahead, lookbehind or code assertions, the parentheses around the
conditional are not needed.
Normally, regexs are a part of Perl expressions.
Code evaluation expressions turn that around by allowing
arbitrary Perl code to be a part of of a regex. A code evaluation
expression is denoted (?{code}), with code a string of Perl
statements.
 There are two possibilities: either the
code expression is used as a conditional in a conditional expression
(?(condition)...), or it is not. If the code expression is a
conditional, the code is evaluated and the result (i.e., the result of
the last statement) is used to determine truth or falsehood. If the
code expression is not used as a conditional, the assertion always
evaluates true and the result is put into the special variable
$^R. The variable $^R can then be used in code expressions later
in the regex. Here are some silly examples:
$x = "abcdef";
$x =~ /abc(?{print "Hi Mom!";})def/; # matches,
# prints 'Hi Mom!'
$x =~ /aaa(?{print "Hi Mom!";})def/; # doesn't match,
# no 'Hi Mom!'
$x =~ /abc(?{print "Hi Mom!";})ddd/; # doesn't match,
# no 'Hi Mom!'
$x =~ /(?{print "Hi Mom!";})/; # matches,
# prints 'Hi Mom!'
$x =~ /(?{$c = 1;})(?{print "$c";})/; # matches,
# prints '1'
$x =~ /(?{$c = 1;})(?{print "$^R";})/; # matches,
# prints '1'
The bit of magic mentioned in the section title occurs when the regex
backtracks in the process of searching for a match. If the regex
backtracks over a code expression and if the variables used within are
localized using local, the changes in the variables produced by the
code expression are undone! Thus, if we wanted to count how many times
a character got matched inside a group, we could use, e.g.,
$x = "aaaa";
$count = 0; # initialize 'a' count
$c = "bob"; # test if $c gets clobbered
$x =~ /(?{local $c = 0;}) # initialize count
( a # match 'a'
(?{local $c = $c + 1;}) # increment count
)* # do this any number of times,
aa # but match 'aa' at the end
(?{$count = $c;}) # copy local $c var into $count
/x;
print "'a' count is $count, \$c variable is '$c'\n";
This prints
'a' count is 2, $c variable is 'bob'
If we replace the (?{local $c = $c + 1;}) with
(?{$c = $c + 1;}) , the variable changes are not undone
during backtracking, and we get
'a' count is 4, $c variable is 'bob'
Note that only localized variable changes are undone. Other side
effects of code expression execution are permanent. Thus
$x = "aaaa";
$x =~ /(a(?{print "Yow\n";}))*aa/;
produces
Yow
Yow
Yow
Yow
The result $^R is automatically localized, so that it will behave
properly in the presence of backtracking.
This example uses a code expression in a conditional to match the
article 'the' in either English or German:
$lang = 'DE'; # use German
...
$text = "das";
print "matched\n"
if $text =~ /(?(?{
$lang eq 'EN'; # is the language English?
})
the | # if so, then match 'the'
(die|das|der) # else, match 'die|das|der'
)
/xi;
Note that the syntax here is (?(?{...})yes-regex|no-regex), not
(?((?{...}))yes-regex|no-regex). In other words, in the case of a
code expression, we don't need the extra parentheses around the
conditional.
If you try to use code expressions with interpolating variables, perl
may surprise you:
$bar = 5;
$pat = '(?{ 1 })';
/foo(?{ $bar })bar/; # compiles ok, $bar not interpolated
/foo(?{ 1 })$bar/; # compile error!
/foo${pat}bar/; # compile error!
$pat = qr/(?{ $foo = 1 })/; # precompile code regex
/foo${pat}bar/; # compiles ok
If a regex has (1) code expressions and interpolating variables,or
(2) a variable that interpolates a code expression, perl treats the
regex as an error. If the code expression is precompiled into a
variable, however, interpolating is ok. The question is, why is this
an error?
The reason is that variable interpolation and code expressions
together pose a security risk. The combination is dangerous because
many programmers who write search engines often take user input and
plug it directly into a regex:
$regex = <>; # read user-supplied regex
$chomp $regex; # get rid of possible newline
$text =~ /$regex/; # search $text for the $regex
If the $regex variable contains a code expression, the user could
then execute arbitrary Perl code. For instance, some joker could
search for system('rm -rf *'); to erase your files. In this
sense, the combination of interpolation and code expressions taints
your regex. So by default, using both interpolation and code
expressions in the same regex is not allowed. If you're not
concerned about malicious users, it is possible to bypass this
security check by invoking use re 'eval' :
use re 'eval'; # throw caution out the door
$bar = 5;
$pat = '(?{ 1 })';
/foo(?{ 1 })$bar/; # compiles ok
/foo${pat}bar/; # compiles ok
Another form of code expression is the pattern code expression .
The pattern code expression is like a regular code expression, except
that the result of the code evaluation is treated as a regular
expression and matched immediately. A simple example is
$length = 5;
$char = 'a';
$x = 'aaaaabb';
$x =~ /(??{$char x $length})/x; # matches, there are 5 of 'a'
This final example contains both ordinary and pattern code
expressions. It detects if a binary string<PHONE_NUMBER>001... has a
Fibonacci spacing 0,1,1,2,3,5,... of the 1's:
$s0 = 0; $s1 = 1; # initial conditions
$x = "1101010010001000001";
print "It is a Fibonacci sequence\n"
if $x =~ /^1 # match an initial '1'
(
(??{'0' x $s0}) # match $s0 of '0'
1 # and then a '1'
(?{
$largest = $s0; # largest seq so far
$s2 = $s1 + $s0; # compute next term
$s0 = $s1; # in Fibonacci sequence
$s1 = $s2;
})
)+ # repeat as needed
$ # that is all there is
/x;
print "Largest sequence matched was $largest\n";
This prints
It is a Fibonacci sequence
Largest sequence matched was 5
Ha! Try that with your garden variety regex package...
Note that the variables $s0 and $s1 are not substituted when the
regex is compiled, as happens for ordinary variables outside a code
expression. Rather, the code expressions are evaluated when perl
encounters them during the search for a match.
The regex without the //x modifier is
/^1((??{'0'x$s0})1(?{$largest=$s0;$s2=$s1+$s0$s0=$s1;$s1=$s2;}))+$/;
and is a great start on an Obfuscated Perl entry :-) When working with
code and conditional expressions, the extended form of regexs is
almost necessary in creating and debugging regexs.
Speaking of debugging, there are several pragmas available to control
and debug regexs in Perl. We have already encountered one pragma in
the previous section, use re 'eval'; , that allows variable
interpolation and code expressions to coexist in a regex. The other
pragmas are
use re 'taint';
$tainted = <>;
@parts = ($tainted =~ /(\w+)\s+(\w+)/; # @parts is now tainted
The taint pragma causes any substrings from a match with a tainted
variable to be tainted as well. This is not normally the case, as
regexs are often used to extract the safe bits from a tainted
variable. Use taint when you are not extracting safe bits, but are
performing some other processing. Both taint and eval pragmas
are lexically scoped, which means they are in effect only until
the end of the block enclosing the pragmas.
use re 'debug';
/^(.*)$/s; # output debugging info
use re 'debugcolor';
/^(.*)$/s; # output debugging info in living color
The global debug and debugcolor pragmas allow one to get
detailed debugging info about regex compilation and
execution. debugcolor is the same as debug, except the debugging
information is displayed in color on terminals that can display
termcap color sequences. Here is example output:
% perl -e 'use re "debug"; "abc" =~ /a*b+c/;'
Compiling REx `a*b+c'
size 9 first at 1
1: STAR(4)
2: EXACT <a>(0)
4: PLUS(7)
5: EXACT <b>(0)
7: EXACT <c>(9)
9: END(0)
floating `bc' at 0..2147483647 (checking floating) minlen 2
Guessing start of match, REx `a*b+c' against `abc'...
Found floating substr `bc' at offset 1...
Guessed: match at offset 0
Matching REx `a*b+c' against `abc'
Setting an EVAL scope, savestack=3
0 <> <abc> | 1: STAR
EXACT <a> can match 1 times out of 32767...
Setting an EVAL scope, savestack=3
1 <a> <bc> | 4: PLUS
EXACT <b> can match 1 times out of 32767...
Setting an EVAL scope, savestack=3
2 <ab> <c> | 7: EXACT <c>
3 <abc> <> | 9: END
Match successful!
Freeing REx: `a*b+c'
If you have gotten this far into the tutorial, you can probably guess
what the different parts of the debugging output tell you. The first
part
Compiling REx `a*b+c'
size 9 first at 1
1: STAR(4)
2: EXACT <a>(0)
4: PLUS(7)
5: EXACT <b>(0)
7: EXACT <c>(9)
9: END(0)
describes the compilation stage. STAR(4) means that there is a
starred object, in this case 'a', and if it matches, goto line 4,
i.e., PLUS(7). The middle lines describe some heuristics and
optimizations performed before a match:
floating `bc' at 0..2147483647 (checking floating) minlen 2
Guessing start of match, REx `a*b+c' against `abc'...
Found floating substr `bc' at offset 1...
Guessed: match at offset 0
Then the match is executed and the remaining lines describe the
process:
Matching REx `a*b+c' against `abc'
Setting an EVAL scope, savestack=3
0 <> <abc> | 1: STAR
EXACT <a> can match 1 times out of 32767...
Setting an EVAL scope, savestack=3
1 <a> <bc> | 4: PLUS
EXACT <b> can match 1 times out of 32767...
Setting an EVAL scope, savestack=3
2 <ab> <c> | 7: EXACT <c>
3 <abc> <> | 9: END
Match successful!
Freeing REx: `a*b+c'
Each step is of the form n <x> <y> , with <x> the
part of the string matched and <y> the part not yet
matched. The | 1: STAR says that perl is at line number 1
n the compilation list above. See
perldebguts/``Debugging regular expressions'' for much more detail.
An alternative method of debugging regexs is to embed print
statements within the regex. This provides a blow-by-blow account of
the backtracking in an alternation:
"that this" =~ m@(?{print "Start at position ", pos, "\n";})
t(?{print "t1\n";})
h(?{print "h1\n";})
i(?{print "i1\n";})
s(?{print "s1\n";})
|
t(?{print "t2\n";})
h(?{print "h2\n";})
a(?{print "a2\n";})
t(?{print "t2\n";})
(?{print "Done at position ", pos, "\n";})
@x;
prints
Start at position 0
t1
h1
t2
h2
a2
t2
Done at position 4
Code expressions, conditional expressions, and independent expressions
are experimental. Don't use them in production code. Yet.
This is just a tutorial. For the full story on perl regular
expressions, see the perlre regular expressions reference page.
For more information on the matching m// and substitution s///
operators, see perlop/``regex Quote-Like Operators''. For
information on the split operation, see perlfunc/split.
For an excellent all-around resource on the care and feeding of
regular expressions, see the book Mastering Regular Expressions by
Jeffrey Friedl (published by O'Reilly, ISBN 1556592-257-3).
Copyright (c) 2000 Mark Kvale
All rights reserved.
This document may be distributed under the same terms as Perl itself.
The inspiration for the stop codon DNA example came from the ZIP
code example in chapter 7 of Mastering Regular Expressions.
The author would like to thank Jeff Pinyan, Andrew Johnson, Peter
Haworth, Ronald J Kimball, and Joe Smith for all their helpful
comments.


detectShift_jeg.Rmd
library(actR)
This vignette shows how actR detects shift (and their statistical significance) for 2 of our example datasets: the geob7926_2 sedimentary record of Kim et al 2012 covering the last deglaciation (roughly 28 to 9ka BP) and the Great Basin tree-ring record of Salzer et al 2013 covering the Common Era at annual resolution.
msGeob <- detectShift(geob7926_2, time.variable.name = "ageEnsemble", vals.variable.name = "SST_from_uk37_ensemble", null.hypothesis.n = 50, summary.bin.step = 500, minimum.segment.length = 3000, n.ens = 50) #> Selected SST_from_uk37_ensemble #> timeUnits is at least partially absent in the input data #> Testing null hypothesis with 50 simulations, each with 50 ensemble members. #> This will probably take about 1 minutes
summary(msGeob) #> GeoB7926_2.Kim.2012 - SST_from_uk37_ensemble: Shift in Mean results #> Searched for Shift in Mean with a minimum segment length of 3000 years, summarizing the results over windows of 500 years. #> #> Overall result: Detected 4 Shift in Mean event(s) that were significant at the alpha < 0.05 level #> # A tibble: 4 x 3 #> time_start time_end empirical_pvalue #> <dbl> <dbl> <dbl> #> 1 9727. 10227. 0 #> 2 25727. 26227. 0.02 #> 3 3727. 4227. 0.04 #> 4 18227. 18727. 0.04 #> Time uncertainty considered? TRUE Time ensemble supplied (n = 100) #> Paleo uncertainty considered? TRUE Values ensemble supplied (n = 2500) #> Error propagation ensemble members = 50 #> Null hypothesis testing ensemble members = 50 #> Members simulated using isospectral method #> #> Parameter choices: #> cpt.fun = changepoint::cpt.mean #> minimum.segment.length = 3000 #> method = AMOC #> penalty = MBIC #> ncpts.max = 1
plot(msGeob)
msGb <- detectShift(great_basin, time.variable.name = "year", vals.variable.name = "trsgi", null.hypothesis.n = 50, summary.bin.step = 1, minimum.segment.length = 50, simulate.time.uncertainty = FALSE, cpt.fun = changepoint::cpt.mean, paleo.uncertainty = 0.1, n.ens = 50) #> Selected trsgi #> Testing null hypothesis with 50 simulations, each with 50 ensemble members. #> This will probably take about 1 minutes
plot(msGb)
vsGb <- detectShift(great_basin, time.variable.name = "year", vals.variable.name = "trsgi", null.hypothesis.n = 50, summary.bin.step = 10, minimum.segment.length = 50, simulate.time.uncertainty = FALSE, paleo.uncertainty = 0.1, cpt.fun = changepoint::cpt.var,surrogate.method = "shuffle", n.ens = 50) #> Selected trsgi #> Testing null hypothesis with 50 simulations, each with 50 ensemble members. #> This will probably take about 1 minutes
plot(vsGb)


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Survey results in for new high school being built in Lee County
by WFTX Digital TeamLEE COUNTY, Fla. — Gateway High School received the most votes for the name of the new high school being built in Lee County.
The school board announced the results yesterday following surveys sent out to the public.
The school is being built on State Road 82, and Griffin Drive is being referred to as MMM.
The school board will make the final decision on the school's name at their meeting on Tuesday.
RELATED STORIES:
PRINCIPAL NAMED FOR UPCOMING HIGH SCHOOL IN LEE COUNTY
BOARD MEMBER WANTS TO NAME NEW LEE COUNTY SCHOOL AFTER MLK
NAME SUGGESTIONS SOUGHT FOR NEW LEE COUNTY HIGH SCHOOL
MEETING ON NEW LEE COUNTY SCHOOL SCHEDULED FOR AUGUST 29TH
ONE WEEK LEFT TO SUGGEST NAMES FOR NEW HIGH SCHOOL IN LEE COUNTY


Home
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The web crawler should parse all types of anchor tags. Among others, the following are supported:
Links with schemes, paths, trailing slashes, and fragments
Links with schemes, paths, and trailing slashes
Links with schemes, paths
Links with schemes
Relative paths
Non-specific schemes
Removes duplicates
Removes duplicates


Keeps track of a number of TCP sessions.
This module will manage a collection of TCP sessions for the same
server. If Port
is 0, the underlying OS assigns an available
port number. If a Name
is not specified, the server will be
named picoSocketServer_
++ the TCP port number that the service
listens to, e.g. picoSocketServer_9923
.
A managed server can be started, stopped, enumerate child sessions, and limit the maximum number of child sessions.
The conventions used by this module look quite different than OTP-based services, due to its origin.| cold_start/7 | |
| server_children/1 | |
| server_children/2 | |
| server_port/1 | |
| server_port/2 | |
| server_status/1 | |
| server_status/2 | |
| start_child/3 | |
| start_raw_server/5 | |
| start_raw_server/7 | |
| start_server/3 | Start a new UBF contract-using server. |
| start_server/4 | |
| stop_server/1 |
cold_start(Parent, Name, Port, Max, Fun, PacketType, PacketSize) -> any()
server_children(Name) -> any()
server_children(Pid, Timeout) -> any()
server_port(Name) -> any()
server_port(Pid, Timeout) -> any()
server_status(Name) -> any()
server_status(Pid, Timeout) -> any()
start_child(Parent, Listen, Fun) -> any()
start_raw_server(Port, Max, Fun, PacketType, PacketSize) -> any()
start_raw_server(Name, Port, Max, SpawnOpts, Fun, PacketType, PacketSize) -> any()
start_server(Port, Max, Fun) -> any()
Start a new UBF contract-using server.
Fun = fun(Socket) -> exit(normal) | exit(socket_closed) | exit({socket_error, Reason}) | exit(timeout). This fun will handle all of the protocol communication for a single TCP session.
A raw server uses packet length 0 (see start_raw_server/5 and start_raw_server/7).start_server(Name, Port, Max, Fun) -> any()
stop_server(Pid) -> any()
Generated EDoc, $Id$


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HOME
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An early draft of another Epicodus project, notable as being the first page I've ever published.


More about me …
I have two Master’s degrees in linguistics, one from UCLA where I had started a PhD and the other from North Carolina State University. I studied laboratory phonology, sociophonetics, and statistical research methods.
I’m originally from Minnesota, but I’ve lived in Wisconsin, France, North Carolina, Pennsylvania, and California (twice!).
My favorite things are: watching horror and scifi movies, bike riding, discovering new places to eat, going to shows (music and comedy) and struggling to decide what I want to learn next.
Here’s where you can find some of my past work (not necessarily reflective of current work):
- My DEV blog posts
- My Medium blog
- Engaging with the data science community on Kaggle
- Writing R or Python code to analyze data
- Organizing community challenges, like launching Kaggle’s Data Science for Good Program
Most of this public work was in my “20% time”. Behind the scenes, I have a lot of fun wearing many hats as a product manager including writing specs, doing user interviews, analyzing data, collaborating with designers and developers, doing research and strategy, writing documentation, getting creative with marketing, etc. I’ve even had the opportunity to play developer when I wrote the server-side code to create Meta Kaggle, Kaggle’s public dataset about … Kaggle.
I live in North Hollywood, CA with my husband and our cat. I currently work at Kaggle (a Google company) as a Product Lead where I’ve worked discontinuously for over 3 years since 2016. Previously I worked at Stack Overflow as a Product Manager for public Q&A.