=head1 DESCRIPTION
This page covers the very basics of understanding, creating and
-using regular expressions ('regexps') in Perl.
+using regular expressions ('regexes') in Perl.
+
=head1 The Guide
=head2 Simple word matching
-The simplest regexp is simply a word, or more generally, a string of
-characters. A regexp consisting of a word matches any string that
+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, C<World> is a regexp and the C<//> enclosing
-C</World/> tells perl to search a string for a match. The operator
-C<=~> associates the string with the regexp match and produces a true
-value if the regexp matched, or false if the regexp did not match. In
+In this statement, C<World> is a regex and the C<//> enclosing
+C</World/> tells Perl to search a string for a match. The operator
+C<=~> 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, C<World> matches the second word in C<"Hello World">, so the
expression is true. This idea has several variations.
print "It doesn't match\n" if "Hello World" !~ /World/;
-The literal string in the regexp can be replaced by a variable:
+The literal string in the regex can be replaced by a variable:
$greeting = "World";
print "It matches\n" if "Hello World" =~ /$greeting/;
"/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin',
# '/' becomes an ordinary char
-Regexps must match a part of the string I<exactly> in order for the
+Regexes must match a part of the string I<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:
+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 B<metacharacters>, are reserved for use in regexp notation.
+called B<metacharacters>, are reserved for use in regex notation.
The metacharacters are
{}[]()^$.|*+?\
"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
+ "/usr/bin/perl" =~ /\/usr\/bin\/perl/; # matches
-In the last regexp, the forward slash C<'/'> is also backslashed,
-because it is used to delimit the regexp.
+In the last regex, the forward slash C<'/'> is also backslashed,
+because it is used to delimit the regex.
Non-printable ASCII characters are represented by B<escape sequences>.
Common examples are C<\t> for a tab, C<\n> for a newline, and C<\r>
escape sequences, e.g., C<\033>, or hexadecimal escape sequences,
e.g., C<\x1B>:
- "1000\t2000" =~ m(0\t2) # matches
- "cat" =~ /\143\x61\x74/ # matches, but a weird way to spell cat
+ "1000\t2000" =~ m(0\t2) # matches
+ "cat" =~ /\143\x61\x74/ # matches in ASCII, but a weird way to spell cat
-Regexps are treated mostly as double quoted strings, so variable
+Regexes are treated mostly as double-quoted strings, so variable
substitution works:
$foo = 'house';
'cathouse' =~ /cat$foo/; # matches
'housecat' =~ /${foo}cat/; # matches
-With all of the regexps above, if the regexp matched anywhere in the
+With all of the regexes above, if the regex matched anywhere in the
string, it was considered a match. To specify I<where> it should
match, we would use the B<anchor> metacharacters C<^> and C<$>. The
anchor C<^> means match at the beginning of the string and the anchor
C<$> means match at the end of the string, or before a newline at the
end of the string. Some examples:
- "housekeeper" =~ /keeper/; # matches
- "housekeeper" =~ /^keeper/; # doesn't match
- "housekeeper" =~ /keeper$/; # matches
- "housekeeper\n" =~ /keeper$/; # matches
+ "housekeeper" =~ /keeper/; # matches
+ "housekeeper" =~ /^keeper/; # doesn't match
+ "housekeeper" =~ /keeper$/; # matches
+ "housekeeper\n" =~ /keeper$/; # matches
+ "housekeeper" =~ /^housekeeper$/; # matches
=head2 Using character classes
A B<character class> allows a set of possible characters, rather than
-just a single character, to match at a particular point in a regexp.
+just a single character, to match at a particular point in a regex.
Character classes are denoted by brackets C<[...]>, with the set of
characters to be possibly matched inside. Here are some examples:
/cat/; # matches 'cat'
- /[bcr]at/; # matches 'bat, 'cat', or 'rat'
+ /[bcr]at/; # matches 'bat', 'cat', or 'rat'
"abc" =~ /[cab]/; # matches 'a'
In the last statement, even though C<'c'> is the first character in
-the class, the earliest point at which the regexp can match is C<'a'>.
+the class, the earliest point at which the regex can match is C<'a'>.
/[yY][eE][sS]/; # match 'yes' in a case-insensitive way
# 'yes', 'Yes', 'YES', etc.
The special character C<^> in the first position of a character class
denotes a B<negated character class>, which matches any character but
-those in the bracket. Both C<[...]> and C<[^...]> must match a
+those in the brackets. Both C<[...]> and C<[^...]> must match a
character, or the match fails. Then
/[^a]at/; # doesn't match 'aat' or 'at', but matches
/[^0-9]/; # matches a non-numeric character
/[a^]at/; # matches 'aat' or '^at'; here '^' is ordinary
-Perl has several abbreviations for common character classes:
+Perl has several abbreviations for common character classes. (These
+definitions are those that Perl uses in ASCII-safe mode with the C</a> modifier.
+Otherwise they could match many more non-ASCII Unicode characters as
+well. See L<perlrecharclass/Backslash sequences> for details.)
=over 4
=item *
-\d is a digit and represents [0-9]
+
+\d is a digit and represents
+
+ [0-9]
=item *
-\s is a whitespace character and represents [\ \t\r\n\f]
+
+\s is a whitespace character and represents
+
+ [\ \t\r\n\f]
=item *
-\w is a word character (alphanumeric or _) and represents [0-9a-zA-Z_]
+
+\w is a word character (alphanumeric or _) and represents
+
+ [0-9a-zA-Z_]
=item *
-\D is a negated \d; it represents any character but a digit [^0-9]
+
+\D is a negated \d; it represents any character but a digit
+
+ [^0-9]
=item *
-\S is a negated \s; it represents any non-whitespace character [^\s]
+
+\S is a negated \s; it represents any non-whitespace character
+
+ [^\s]
=item *
-\W is a negated \w; it represents any non-word character [^\w]
+
+\W is a negated \w; it represents any non-word character
+
+ [^\w]
=item *
+
The period '.' matches any character but "\n"
=back
=head2 Matching this or that
-We can match
match different character strings with the B<alternation>
-metacharacter C<'|'>. To match C<dog> or C<cat>, we form the regexp
-C<dog|cat>. As before, perl will try to match the regexp at the
+We can match different character strings with the B<alternation>
+metacharacter C<'|'>. To match C<dog> or C<cat>, we form the regex
+C<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, C<dog>. If
-C<dog> doesn't match, perl will then try the next alternative, C<cat>.
-If C<cat> doesn't match either, then the match fails and perl moves to
+Perl will first try to match the first alternative, C<dog>. If
+C<dog> doesn't match, Perl will then try the next alternative, C<cat>.
+If C<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 C<dog> is the first alternative in the second regexp,
+Even though C<dog> is the first alternative in the second regex,
C<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
-regexp match to succeed wil be the one that matches. Here, all the
-alternatives match at the first string position, so th first matches.
+regex match to succeed will be the one that matches. Here, all the
+alternatives match at the first string position, so the first matches.
=head2 Grouping things and hierarchical matching
-The B<grouping> metacharacters C<()> allow a part of a regexp to be
-treated as a single unit. Parts of a regexp are grouped by enclosing
-them in parentheses. The regexp C<house(cat|keeper)> means match
+The B<grouping> metacharacters C<()> 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 C<house(cat|keeper)> means match
C<house> followed by either C<cat> or C<keeper>. Some more examples
are
$minutes = $2;
$seconds = $3;
-In list context, a match C</regexp/ with groupings will return the
+In list context, a match C</regex/> with groupings will return the
list of matched values C<($1,$2,...)>. So we could rewrite it as
($hours, $minutes, $second) = ($time =~ /(\d\d):(\d\d):(\d\d)/);
-If the groupings in a regexp are nested, C<$1> gets the group with the
+If the groupings in a regex are nested, C<$1> gets the group with the
leftmost opening parenthesis, C<$2> the next opening parenthesis,
-etc. For example, here is a complex regexp and the matching variables
+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 C<$1>, C<$2>, ... are
-the B<backreferences> C<\1>, C<\2>, ... Backreferences are
-matching variables that can be used I<inside> a regexp:
+the B<backreferences> C<\g1>, C<\g2>, ... Backreferences are
+matching variables that can be used I<inside> a regex:
- /(\w\w\w)\s\1/; # find sequences like 'the the' in string
+ /(\w\w\w)\s\g1/; # find sequences like 'the the' in string
-C<$1>, C<$2>, ... should only be used outside of a regexp, and C<\1>,
-C<\2>, ... only inside a regexp.
+C<$1>, C<$2>, ... should only be used outside of a regex, and C<\g1>,
+C<\g2>, ... only inside a regex.
=head2 Matching repetitions
The B<quantifier> metacharacters C<?>, C<*>, C<+>, and C<{}> allow us
-to determine the number of repeats of a portion of a regexp we
+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:
=over 4
-=item * C<a?> = match 'a' 1 or 0 times
+=item *
+
+C<a?> = match 'a' 1 or 0 times
+
+=item *
+
+C<a*> = match 'a' 0 or more times, i.e., any number of times
+
+=item *
-=item * C<a*> = match 'a' 0 or more times, i.e., any number of times
+C<a+> = match 'a' 1 or more times, i.e., at least once
-=item * C<a+> = match 'a' 1 or more times, i.e., at least once
+=item *
-
=item * C<a{n,m}> = match at least C<n> times, but not more than C<m>
+C<a{n,m}> = match at least C<n> times, but not more than C<m>
times.
-=item * C<a{n,}> = match at least C<n> or more times
+=item *
+
+C<a{n,}> = match at least C<n> or more times
+
+=item *
-
=item * C<a{n}> = match exactly C<n> times
+C<a{n}> = match exactly C<n> times
=back
/[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
+ /(\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 regexp to match. So we have
+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 C<.*> grabs as much of the string as possible
-while still having the regexp match. The second quantifier C<.*> has
+while still having the regex match. The second quantifier C<.*> has
no string left to it, so it matches 0 times.
=head2 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 C<$pattern> each time through the loop. If
-C<$pattern> won't be changing, use the C<//o> modifier, to only
-perform variable substitutions once. If you don't want any
-substitutions at all, use the special delimiter C<m''>:
-
- $pattern = 'Seuss';
- m'$pattern'; # matches '$pattern', not 'Seuss'
-
+operators.
The global modifier C<//g> allows the matching operator to match
within a string as many times as possible. In scalar context,
successive matches against a string will have C<//g> jump from match
A failed match or changing the target string resets the position. If
you don't want the position reset after failure to match, add the
-C<//c>, as in C</regexp/gc>.
+C<//c>, as in C</regex/gc>.
In list context, C<//g> returns a list of matched groupings, or if
-there are no groupings, a list of matches to the whole regexp. So
+there are no groupings, a list of matches to the whole regex. So
@words = ($x =~ /(\w+)/g); # matches,
# $word[0] = 'cat'
=head2 Search and replace
-Search and replace is perform using C<s/regexp/replacement/modifiers>.
-The C<replacement> is a Perl double quoted string that replaces in the
-string whatever is matched with the C<regexp>. The operator C<=~> is
+Search and replace is performed using C<s/regex/replacement/modifiers>.
+The C<replacement> is a Perl double-quoted string that replaces in the
+string whatever is matched with the C<regex>. The operator C<=~> is
also used here to associate a string with C<s///>. If matching
-against C<$_>, the S<C<$_ =~> > can be dropped. If there is a match,
-C<s///> returns the number of substitutions made, otherwise it returns
+against C<$_>, the S<C<$_ =~>> can be dropped. If there is a match,
+C<s///> returns the number of substitutions made; otherwise it returns
false. Here are a few examples:
$x = "Time to feed the cat!";
With the C<s///> operator, the matched variables C<$1>, C<$2>, etc.
are immediately available for use in the replacement expression. With
the global modifier, C<s///g> will search and replace all occurrences
-of the regexp in the string:
+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; # $x contains "I batted four for four"
+The non-destructive modifier C<s///r> causes the result of the substitution
+to be returned instead of modifying C<$_> (or whatever variable the
+substitute was bound to with C<=~>):
+
+ $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 C<s///e> wraps an C<eval{...}> around the
replacement string and the evaluated result is substituted for the
-matched substring. This counts character frequencies in a line:
+matched substring. Some examples:
- $x = "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);
+ # 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"
-This prints
+ # convert percentage to decimal
+ $x = "A 39% hit rate";
+ $x =~ s!(\d+)%!$1/100!e; # $x contains "A 0.39 hit rate"
- frequency of 't' is 2
- frequency of 'e' is 1
- frequency of ' ' is 1
- frequency of 'h' is 1
- frequency of 'a' is 1
- frequency of 'c' is 1
-
-C<s///> can use other delimiters, such as C<s!!!> and C<s{}{}>, and
-even C<s{}//>. If single quotes are used C<s'''>, then the regexp and
-replacement are treated as single quoted strings.
+The last example shows that C<s///> can use other delimiters, such as
+C<s!!!> and C<s{}{}>, and even C<s{}//>. If single quotes are used
+C<s'''>, then the regex and replacement are treated as single-quoted
+strings.
=head2 The split operator
-C<split /regexp/, string> splits C<string> into a list of substrings
-and returns that list. The regexp determines the character sequence
+C<split /regex/, string> splits C<string> into a list of substrings
+and returns that list. The regex determines the character sequence
that C<string> is split with respect to. 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'
+ @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 regexp C<//> is used, the string is split into individual
-characters. If the regexp has groupings, then list produced contains
+If the empty regex C<//> 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[3] = '/'
# $parts[4] = 'bin'
-Since the first character of $x matched the regexp, C<split> prepended
+Since the first character of $x matched the regex, C<split> prepended
an empty initial element to the list.
=head1 BUGS
=head1 SEE ALSO
This is just a quick start guide. For a more in-depth tutorial on
-regex
ps, see L<perlretut> and for the reference page, see L<perlre>.
+regex
es, see L<perlretut> and for the reference page, see L<perlre>.
=head1 AUTHOR AND COPYRIGHT
This document may be distributed under the same terms as Perl itself.
+=head2 Acknowledgments
+
+The author would like to thank Mark-Jason Dominus, Tom Christiansen,
+Ilya Zakharevich, Brad Hughes, and Mike Giroux for all their helpful
+comments.
+
=cut
https://perl5.git.perl.org / perl5.git / blobdiff