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perlrequick - Perl regular expressions quick start |
perlrequick - Perl regular expressions quick start
This page covers the very basics of understanding, creating and using regular expressions ('regexes') in Perl.
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:
$_ = "Hello World";
print "It matches\n" if /World/;
Finally, the // default delimiters for a match can be changed to
arbitrary delimiters by putting an 'm' out front:
"Hello World" =~ m!World!; # matches, delimited by '!'
"Hello World" =~ m{World}; # matches, note the matching '{}'
"/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin',
# '/' becomes an ordinary char
Regexes must match a part of the string exactly in order for the statement to be true:
"Hello World" =~ /world/; # doesn't match, case sensitive
"Hello World" =~ /o W/; # matches, ' ' is an ordinary char
"Hello World" =~ /World /; # doesn't match, no ' ' at end
perl will always match at the earliest possible point in the string:
"Hello World" =~ /o/; # matches 'o' in 'Hello'
"That hat is red" =~ /hat/; # matches 'hat' in 'That'
Not all characters can be used 'as is' in a match. Some characters, called metacharacters, are reserved for use in regex notation. The metacharacters are
{}[]()^$.|*+?\
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\/local\/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 examples:
"housekeeper" =~ /keeper/; # matches
"housekeeper" =~ /^keeper/; # doesn't match
"housekeeper" =~ /keeper$/; # matches
"housekeeper\n" =~ /keeper$/; # matches
"housekeeper" =~ /^housekeeper$/; # matches
A character class allows a set of possible characters, rather than
just a single character, to match at a particular point in a regex.
Character classes are denoted by brackets [...], with the set of
characters to be possibly matched inside. Here are some examples:
/cat/; # matches 'cat'
/[bcr]at/; # matches 'bat', 'cat', or 'rat'
"abc" =~ /[cab]/; # matches 'a'
In the last statement, even though 'c' is the first character in
the class, the earliest point at which the regex can match is 'a'.
/[yY][eE][sS]/; # match 'yes' in a case-insensitive way
# 'yes', 'Yes', 'YES', etc.
/yes/i; # also match 'yes' in a case-insensitive way
The last example shows a match with an 'i' modifier, which makes
the match case-insensitive.
Character classes also have ordinary and special characters, but the
sets of ordinary and special characters inside a character class are
different than those outside a character class. The special
characters for a character class are -]\^$ 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 [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.
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
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.'
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.
We can match match different character strings with 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 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 wil be the one that matches. Here, all the alternatives match at the first string position, so th first matches.
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
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.
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:
a? = match 'a' 1 or 0 times
a* = match 'a' 0 or more times, i.e., any number of times
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.
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 '$pattern', not 'Seuss'
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 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 expression. 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/; # $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 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.
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 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'
Since the first character of $x matched the regex, split prepended
an empty initial element to the list.
None.
This is just a quick start guide. For a more in-depth tutorial on regexes, see the perlretut manpage and for the reference page, see the perlre manpage.
Copyright (c) 2000 Mark Kvale All rights reserved.
This document may be distributed under the same terms as Perl itself.
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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perlrequick - Perl regular expressions quick start |