MirOS Manual: perlrequick(1)


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 "!~" 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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