PERLREQUICK(1) Perl Programmers Reference Guide PERLREQUICK(1)NAMEperlrequick - 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 "!~" opera-
tor:
print "It doesn't match\n" if "Hello World" !~ /World/;
The literal string in the regex can be replaced by a vari-
able:
$greeting = "World";
print "It matches\n" if "Hello World" =~ /$greeting/;
If you're matching against $_, the "$_ =~" part can be omit-
ted:
$_ = "Hello World";
print "It matches\n" if /World/;
Finally, the "//" default delimiters for a match can be
changed to arbitrary delimiters by putting an 'm' out front:
"Hello World" =~ m!World!; # matches, delimited by '!'
"Hello World" =~ m{World}; # matches, note the matching '{}'
"/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin',
# '/' becomes an ordinary char
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Regexes must match a part of the string exactly in order for
the statement to be true:
"Hello World" =~ /world/; # doesn't match, case sensitive
"Hello World" =~ /o W/; # matches, ' ' is an ordinary char
"Hello World" =~ /World /; # doesn't match, no ' ' at end
perl will always match at the earliest possible point in the
string:
"Hello World" =~ /o/; # matches 'o' in 'Hello'
"That hat is red" =~ /hat/; # matches 'hat' in 'That'
Not all characters can be used 'as is' in a match. Some
characters, called metacharacters, are reserved for use in
regex notation. The metacharacters are
{}[]()^$.|*+?\
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 +
'C:\WIN32' =~ /C:\\WIN/; # matches
"/usr/bin/perl" =~ /\/usr\/bin\/perl/; # matches
In the last regex, the forward slash '/' is also
backslashed, because it is used to delimit the regex.
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, 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 "$" means match at the end of the
string, or before a newline at the end of the string. Some
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examples:
"housekeeper" =~ /keeper/; # matches
"housekeeper" =~ /^keeper/; # doesn't match
"housekeeper" =~ /keeper$/; # 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 particu-
lar 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 charac-
ter 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 "[0123456789]" and
"[abc...xyz]" become the svelte "[0-9]" and "[a-z]":
/item[0-9]/; # matches 'item0' or ... or 'item9'
/[0-9a-fA-F]/; # matches a hexadecimal digit
If '-' is the first or last character in a character class,
it is treated as an ordinary character.
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The special character "^" in the first position of a charac-
ter 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
Perl has several abbreviations for common character classes:
+ \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.'
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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.
Matching this or that
We can match different character strings with the alterna-
tion 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"
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.
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"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 "\1", "\2", ... Backreferences are matching
variables that can be used inside a regex:
/(\w\w\w)\s\1/; # find sequences like 'the the' in string
$1, $2, ... should only be used outside of a regex, and
"\1", "\2", ... 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 group-
ing that we want to specify. They have the following mean-
ings:
+ "a?" = match 'a' 1 or 0 times
+ "a*" = match 'a' 0 or more times, i.e., any number of
times
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+ "a+" = match 'a' 1 or more times, i.e., at least once
+ "a{n,m}" = match at least "n" times, but not more than
"m" times.
+ "a{n,}" = match at least "n" or more times
+ "a{n}" = match exactly "n" times
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
$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. In the code
$pattern = 'Seuss';
while (<>) {
print if /$pattern/;
}
perl has to re-evaluate $pattern each time through the loop.
If $pattern won't be changing, use the "//o" modifier, to
only perform variable substitutions once. If you don't want
any substitutions at all, use the special delimiter "m''":
@pattern = ('Seuss');
m/@pattern/; # matches 'Seuss'
m'@pattern'; # matches the literal string '@pattern'
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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 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!"
$y = "'quoted words'";
$y =~ s/^'(.*)'$/$1/; # strip single quotes,
# $y contains "quoted words"
With the "s///" operator, the matched variables $1, $2, etc.
are immediately available for use in the replacement expres-
sion. With the global modifier, "s///g" will search and
replace all occurrences of the regex in the string:
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$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 evaluation modifier "s///e" wraps an "eval{...}" around
the replacement string and the evaluated result is substi-
tuted 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 sub-
strings 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";
@parts = split m!(/)!, $x; # $parts[0] = ''
# $parts[1] = '/'
# $parts[2] = 'usr'
# $parts[3] = '/'
# $parts[4] = 'bin'
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Since the first character of $x matched the regex, "split"
prepended an empty initial element to the list.
BUGS
None.
SEE ALSO
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.
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