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diff --git a/Master/tlpkg/tlperl/lib/pods/perlrequick.pod b/Master/tlpkg/tlperl/lib/pods/perlrequick.pod new file mode 100644 index 00000000000..4b5e19a0fb1 --- /dev/null +++ b/Master/tlpkg/tlperl/lib/pods/perlrequick.pod @@ -0,0 +1,516 @@ +=head1 NAME + +perlrequick - Perl regular expressions quick start + +=head1 DESCRIPTION + +This page covers the very basics of understanding, creating and +using regular expressions ('regexes') in Perl. + + +=head1 The Guide + +=head2 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, 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. + +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 C<!~> 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 C<$_>, the C<$_ =~> part can be omitted: + + $_ = "Hello World"; + print "It matches\n" if /World/; + +Finally, the C<//> default delimiters for a match can be changed to +arbitrary delimiters by putting an C<'m'> out front: + + "Hello World" =~ m!World!; # matches, delimited by '!' + "Hello World" =~ m{World}; # matches, note the matching '{}' + "/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin', + # '/' becomes an ordinary char + +Regexes must match a part of the string I<exactly> in order for the +statement to be true: + + "Hello World" =~ /world/; # doesn't match, case sensitive + "Hello World" =~ /o W/; # matches, ' ' is an ordinary char + "Hello World" =~ /World /; # doesn't match, no ' ' at end + +perl will always match at the earliest possible point in the string: + + "Hello World" =~ /o/; # matches 'o' in 'Hello' + "That hat is red" =~ /hat/; # matches 'hat' in 'That' + +Not all characters can be used 'as is' in a match. Some characters, +called B<metacharacters>, are reserved for use in regex notation. +The metacharacters are + + {}[]()^$.|*+?\ + +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 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> +for a carriage return. Arbitrary bytes are represented by octal +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 in ASCII, 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' =~ /${foo}cat/; # matches + +With all of the regexes above, if the regex matched anywhere in the +string, it was considered a match. To specify I<where> it should +match, we would use the B<anchor> metacharacters C<^> and C<$>. 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$/; # matches + "housekeeper" =~ /^housekeeper$/; # 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 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' + "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 regex can match is C<'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 C<'i'> B<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 C<-]\^$> 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 C<'-'> acts as a range operator within character +classes, so that the unwieldy C<[0123456789]> and C<[abc...xyz]> +become the svelte C<[0-9]> and C<[a-z]>: + + /item[0-9]/; # matches 'item0' or ... or 'item9' + /[0-9a-fA-F]/; # matches a hexadecimal digit + +If C<'-'> is the first or last character in a character class, it is +treated as an ordinary character. + +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 brackets. Both C<[...]> and C<[^...]> 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: + +=over 4 + +=item * + +\d is a digit and represents + + [0-9] + +=item * + +\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_] + +=item * + +\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] + +=item * + +\W is a negated \w; it represents any non-word character + + [^\w] + +=item * + +The period '.' matches any character but "\n" + +=back + +The C<\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 S<B<word anchor> > C<\b> matches a boundary between a word +character and a non-word character C<\w\W> or C<\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. + +=head2 Matching this or that + +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 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 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 +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 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 + + /(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 + +=head2 Extracting matches + +The grouping metacharacters C<()> 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 C<$1>, C<$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 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 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 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 regex: + + /(\w\w\w)\s\1/; # find sequences like 'the the' in string + +C<$1>, C<$2>, ... should only be used outside of a regex, and C<\1>, +C<\2>, ... 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 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' 0 or more times, i.e., any number of times + +=item * + +C<a+> = match 'a' 1 or more times, i.e., at least once + +=item * + +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 exactly C<n> times + +=back + +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 C<.*> grabs as much of the string as possible +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 'Seuss' + m'@pattern'; # matches the literal string '@pattern' + +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 +to match, keeping track of position in the string as it goes along. +You can get or set the position with the C<pos()> function. +For example, + + $x = "cat dog house"; # 3 words + while ($x =~ /(\w+)/g) { + print "Word is $1, ends at position ", pos $x, "\n"; + } + +prints + + Word is cat, ends at position 3 + Word is dog, ends at position 7 + Word is house, ends at position 13 + +A failed match or changing the target string resets the position. If +you don't want the position reset after failure to match, add the +C<//c>, as in C</regex/gc>. + +In list context, C<//g> returns a list of matched groupings, or if +there are no groupings, a list of matches to the whole regex. So + + @words = ($x =~ /(\w+)/g); # matches, + # $word[0] = 'cat' + # $word[1] = 'dog' + # $word[2] = 'house' + +=head2 Search and replace + +Search and replace is performed using C<s/regex/replacement/modifiers>. +The C<replacement> is a Perl double quoted string that replaces in the +string whatever is matched with the C<regex>. The operator C<=~> is +also used here to associate a string with C<s///>. If matching +against C<$_>, the S<C<$_ =~> > can be dropped. If there is a match, +C<s///> returns the number of substitutions made, otherwise it returns +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 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 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 C<s///e> wraps an C<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 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 /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"; + @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 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 = split m!(/)!, $x; # $parts[0] = '' + # $parts[1] = '/' + # $parts[2] = 'usr' + # $parts[3] = '/' + # $parts[4] = 'bin' + +Since the first character of $x matched the regex, C<split> prepended +an empty initial element to the list. + +=head1 BUGS + +None. + +=head1 SEE ALSO + +This is just a quick start guide. For a more in-depth tutorial on +regexes, see L<perlretut> and for the reference page, see L<perlre>. + +=head1 AUTHOR AND COPYRIGHT + +Copyright (c) 2000 Mark Kvale +All rights reserved. + +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 + |