PERLREQUICK(1) Perl Programmers Reference Guide PERLREQUICK(1)
NAME
perlrequick - Perl regular expressions quick start
DESCRIPTION
This page covers the very basics of understanding, creating and using regular expressions ('regexes') in Perl.
The Guide
Simple word matching
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
In this statement, "World" is a regex 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. This idea has several variations.
Expressions like this are useful in conditionals:
print "It matches\n" if "Hello World" =~ /World/;
The sense of the match can be reversed by using "!~" operator:
print "It doesn't match\n" if "Hello World" !~ /World/;
The literal string in the regex can be replaced by a variable:
$greeting = "World";
print "It matches\n" if "Hello World" =~ /$greeting/;
If you're matching against $_, the "$_ =~" part can be omitted:
$_ = "Hello World";
print "It matches\n" if /World/;
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
Regexes must match a part of the string exactly in order for the statement to be true:
"Hello World" =~ /world/; # doesn't match, case sensitive
"Hello World" =~ /o W/; # matches, ' ' is an ordinary char
"Hello World" =~ /World /; # doesn't match, no ' ' at end
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'
Not all characters can be used 'as is' in a match. Some characters, called metacharacters, are reserved for
Non-printable ASCII characters are represented by escape sequences. Common examples are "\t" for a tab, "\n"
for a newline, and "\r" for a carriage return. Arbitrary bytes are represented by octal escape sequences,
e.g., "\033", or hexadecimal escape sequences, e.g., "\x1B":
"1000\t2000" =~ m(0\t2) # matches
"cat" =~ /\143\x61\x74/ # matches in ASCII, but a weird way to spell cat
Regexes are treated mostly as double-quoted strings, so variable substitution works:
$foo = 'house';
'cathouse' =~ /cat$foo/; # matches
'housecat' =~ /${foo}cat/; # matches
With all of the regexes above, if the regex matched anywhere in the string, it was considered a match. To
specify where it should match, we would use the anchor metacharacters "^" and "$". The anchor "^" means match
at the beginning of the string and the anchor "$" means match at the end of the string, or before a newline at
the end of the string. Some examples:
"housekeeper" =~ /keeper/; # matches
"housekeeper" =~ /^keeper/; # doesn't match
"housekeeper" =~ /keeper$/; # matches
"housekeeper\n" =~ /keeper$/; # matches
"housekeeper" =~ /^housekeeper$/; # matches
Using character classes
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'
"abc" =~ /[cab]/; # matches 'a'
In the last statement, even though 'c' is the first character in the class, the earliest point at which the
regex can match is 'a'.
/[yY][eE][sS]/; # match 'yes' in a case-insensitive way
# 'yes', 'Yes', 'YES', etc.
/yes/i; # also match 'yes' in a case-insensitive way
The last example shows a match with an 'i' modifier, which makes the match case-insensitive.
Character classes also have ordinary and special characters, 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 "-]\^$" and are matched using an escape:
/[\]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 special character '-' acts as a range operator within character classes, so that the unwieldy
# all other 'bat', 'cat, '0at', '%at', etc.
/[^0-9]/; # matches a non-numeric character
/[a^]at/; # matches 'aat' or '^at'; here '^' is ordinary
Perl has several abbreviations for common character classes. (These definitions are those that Perl uses in
ASCII-safe mode with the "/a" modifier. Otherwise they could match many more non-ASCII Unicode characters as
well. See "Backslash sequences" in perlrecharclass for details.)
· \d is a digit and represents
[0-9]
· \s is a whitespace character and represents
[\ \t\r\n\f]
· \w is a word character (alphanumeric or _) and represents
[0-9a-zA-Z_]
· \D is a negated \d; it represents any character but a digit
[^0-9]
· \S is a negated \s; it represents any non-whitespace character
[^\s]
· \W is a negated \w; it represents any non-word character
[^\w]
· The period '.' matches any character but "\n"
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.'
The word anchor "\b" matches a boundary between a word character and a non-word character "\w\W" or "\W\w":
$x = "Housecat catenates house and cat";
$x =~ /\bcat/; # matches cat in 'catenates'
$x =~ /cat\b/; # matches cat in 'housecat'
$x =~ /\bcat\b/; # matches 'cat' at end of string
In the last example, the end of the string is considered a word boundary.
"cats" =~ /c|ca|cat|cats/; # matches "c"
"cats" =~ /cats|cat|ca|c/; # matches "cats"
At a given character position, the first alternative that allows the regex match to succeed will be the one
that matches. Here, all the alternatives match at the first string position, so the first matches.
Grouping things and hierarchical matching
The grouping metacharacters "()" allow a part of a regex to be treated as a single unit. Parts of a regex are
grouped by enclosing them in parentheses. 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'
/(^a|b)c/; # matches 'ac' at start of string or 'bc' anywhere
/house(cat|)/; # matches either 'housecat' or 'house'
/house(cat(s|)|)/; # matches either 'housecats' or 'housecat' or
# 'house'. Note groups can be nested.
"20" =~ /(19|20|)\d\d/; # matches the null alternative '()\d\d',
# because '20\d\d' can't match
Extracting matches
The grouping metacharacters "()" also allow the extraction of the parts of a string that matched. 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;
In list context, a match "/regex/" with groupings will return the list of matched values "($1,$2,...)". So we
could rewrite it as
($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
Associated with the matching variables $1, $2, ... are the backreferences "\g1", "\g2", ... Backreferences
are matching variables that can be used inside a regex:
/(\w\w\w)\s\g1/; # find sequences like 'the the' in string
$1, $2, ... should only be used outside of a regex, and "\g1", "\g2", ... only inside a regex.
Matching repetitions
The quantifier metacharacters "?", "*", "+", and "{}" 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+\g1/; # match doubled words of arbitrary length
$year =~ /^\d{2,4}$/; # 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
These quantifiers will try to match as much of the string as possible, while still allowing the regex to
match. So we have
$x = 'the cat in the hat';
$x =~ /^(.*)(at)(.*)$/; # matches,
# $1 = 'the cat in the h'
# $2 = 'at'
# $3 = '' (0 matches)
The first quantifier ".*" grabs as much of the string as possible while still having the regex match. The
second quantifier ".*" has no string left to it, so it matches 0 times.
More matching
There are a few more things you might want to know about matching operators. The global modifier "//g" allows
the matching operator to match within a string as many times as possible. In scalar context, successive
matches 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. For example,
$x = "cat dog house"; # 3 words
while ($x =~ /(\w+)/g) {
print "Word is $1, ends at position ", pos $x, "\n";
}
prints
Word is cat, ends at position 3
Word is dog, ends at position 7
Word is house, ends at position 13
A failed match or changing the target string resets the position. If you don't want the position reset after
failure to match, add the "//c", as in "/regex/gc".
In list context, "//g" returns a list of matched groupings, or if there are no groupings, a list of matches to
the whole regex. So
@words = ($x =~ /(\w+)/g); # matches,
# $word[0] = 'cat'
# $word[1] = 'dog'
# $word[2] = 'house'
Search and replace
Search and replace is performed using "s/regex/replacement/modifiers". 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
$x = "I batted 4 for 4";
$x =~ s/4/four/; # $x contains "I batted four for 4"
$x = "I batted 4 for 4";
$x =~ s/4/four/g; # $x contains "I batted four for four"
The non-destructive modifier "s///r" causes the result of the substitution to be returned instead of modifying
$_ (or whatever variable the substitute was bound to with "=~"):
$x = "I like dogs.";
$y = $x =~ s/dogs/cats/r;
print "$x $y\n"; # prints "I like dogs. I like cats."
$x = "Cats are great.";
print $x =~ s/Cats/Dogs/r =~ s/Dogs/Frogs/r =~ s/Frogs/Hedgehogs/r, "\n";
# prints "Hedgehogs are great."
@foo = map { s/[a-z]/X/r } qw(a b c 1 2 3);
# @foo is now qw(X X X 1 2 3)
The evaluation modifier "s///e" wraps an "eval{...}" around the replacement string and the evaluated result is
substituted for the matched substring. Some examples:
# reverse all the words in a string
$x = "the cat in the hat";
$x =~ s/(\w+)/reverse $1/ge; # $x contains "eht tac ni eht tah"
# convert percentage to decimal
$x = "A 39% hit rate";
$x =~ s!(\d+)%!$1/100!e; # $x contains "A 0.39 hit rate"
The last example shows that "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.
The split operator
"split /regex/, string" splits "string" into a list of substrings and returns that list. The regex determines
the character sequence that "string" is split with respect to. For example, to split a string into words, use
$x = "Calvin and Hobbes";
@word = split /\s+/, $x; # $word[0] = 'Calvin'
# $word[1] = 'and'
# $word[2] = 'Hobbes'
To extract a comma-delimited list of numbers, use
$x = "1.618,2.718, 3.142";
@const = split /,\s*/, $x; # $const[0] = '1.618'
# $const[1] = '2.718'
# $const[2] = '3.142'
If the empty regex "//" is used, the string is split into individual characters. If the regex has groupings,
then the list produced contains the matched substrings from the groupings as well:
$x = "/usr/bin";
This is just a quick start guide. For a more in-depth tutorial on regexes, see perlretut and for the
reference page, see perlre.
AUTHOR AND COPYRIGHT
Copyright (c) 2000 Mark Kvale All rights reserved.
This document may be distributed under the same terms as Perl itself.
Acknowledgments
The author would like to thank Mark-Jason Dominus, Tom Christiansen, Ilya Zakharevich, Brad Hughes, and Mike
Giroux for all their helpful comments.
perl v5.16.3 2013-03-04 PERLREQUICK(1)
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