perlrequick
DESCRIPTION
This page covers the very basics of understanding, creat
ing 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 "//" enclos
ing "/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 expres
sion 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 "!~" oper
ator:
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
omitted:
$_ = "Hello World";
print "It matches\n" if /World/;
Finally, the "//" default delimiters for a match can be
changed to arbitrary delimiters by putting an 'm' out
front:
"Hello World" =~ m!World!; # matches, delimited by '!'
"Hello World" =~ m{World}; # matches, note the matching '{}'
"/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin',
# '/' becomes an ordinary char
Regexes must match a part of the string exactly in order
regex notation. The metacharacters are
{}[]()^$.|*+?\
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 back
slashed, 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' =~ /${foo}cat/; # matches
With all of the regexes above, if the regex matched any
where in the string, it was considered a match. To spec
ify where it should match, we would use the anchor
metacharacters "^" and "$". 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 partic
ular point in a regex. Character classes are denoted by
brackets "[...]", with the set of characters to be possi
bly matched inside. Here are some examples:
makes the match case-insensitive.
Character classes also have ordinary and special charac
ters, 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 char
acter 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 repre
sents
[0-9a-zA-Z_]
· \D is a negated \d; it represents any character but a
· 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.
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
"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 vari
ables $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 match
ing 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
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
# 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" modi
fier, 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 $x, "\n";
}
@words = ($x =~ /(\w+)/g); # matches,
# $word[0] = 'cat'
# $word[1] = 'dog'
# $word[2] = 'house'
Search and replace
Search and replace is performed using "s/regex/replace
ment/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 delim
iters, such as "s!!!" and "s{}{}", and even "s{}//". If
single quotes are used "s'''", then the regex and replace
ment are treated as single quoted strings.
The split operator
"split /regex/, string" splits "string" into a list of
$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'
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.
perl v5.8.1 2003-09-02 PERLREQUICK(1)
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