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diff --git a/pod/perlrequick.pod b/pod/perlrequick.pod index d151e26a0c..a14229c303 100644 --- a/pod/perlrequick.pod +++ b/pod/perlrequick.pod @@ -5,22 +5,23 @@ perlrequick - Perl regular expressions quick start =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 +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 regexp match and produces a true -value if the regexp matched, or false if the regexp did not match. In +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. @@ -32,7 +33,7 @@ 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 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/; @@ -50,7 +51,7 @@ arbitrary delimiters by putting an C<'m'> out front: "/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 @@ -63,7 +64,7 @@ perl will always match at the earliest possible point in the string: "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 {}[]()^$.|*+?\ @@ -75,8 +76,8 @@ A metacharacter can be matched by putting a backslash before it: 'C:\WIN32' =~ /C:\\WIN/; # matches "/usr/bin/perl" =~ /\/usr\/local\/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> @@ -87,38 +88,39 @@ e.g., C<\x1B>: "1000\t2000" =~ m(0\t2) # matches "cat" =~ /\143\x61\x74/ # matches, 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. @@ -151,7 +153,7 @@ 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 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 @@ -211,8 +213,8 @@ boundary. =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 +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>. @@ -222,21 +224,21 @@ 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 +regex match to succeed wil be the one that matches. Here, all the alternatives match at the first string position, so th 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 @@ -263,14 +265,14 @@ They can be used just as ordinary variables: $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))/; @@ -278,17 +280,17 @@ indicated below it: 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: +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 regexp, and C<\1>, -C<\2>, ... only inside a regexp. +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 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: @@ -320,15 +322,16 @@ Here are some examples: $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' # $3 = '' (0 matches) The first quantifier C<.*> grabs as much of the string as possible -while still having the regexp match. The second quantifier C<.*> has +while still having the regex match. The second quantifier C<.*> has no string left to it, so it matches 0 times. =head2 More matching @@ -369,10 +372,10 @@ prints A failed match or changing the target string resets the position. If you don't want the position reset after failure to match, add the -C<//c>, as in C</regexp/gc>. +C<//c>, as in C</regex/gc>. In list context, C<//g> returns a list of matched groupings, or if -there are no groupings, a list of matches to the whole regexp. So +there are no groupings, a list of matches to the whole regex. So @words = ($x =~ /(\w+)/g); # matches, # $word[0] = 'cat' @@ -381,9 +384,9 @@ there are no groupings, a list of matches to the whole regexp. So =head2 Search and replace -Search and replace is perform using C<s/regexp/replacement/modifiers>. +Search and replace is performed using C<s/regex/replacement/modifiers>. The C<replacement> is a Perl double quoted string that replaces in the -string whatever is matched with the C<regexp>. The operator C<=~> is +string whatever is matched with the C<regex>. The operator C<=~> is also used here to associate a string with C<s///>. If matching against C<$_>, the S<C<$_ =~> > can be dropped. If there is a match, C<s///> returns the number of substitutions made, otherwise it returns @@ -398,7 +401,7 @@ false. Here are a few examples: 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" @@ -407,40 +410,42 @@ of the regexp in the string: 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: - - $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); +matched substring. Some examples: -This prints + # 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" - 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 + # convert percentage to decimal + $x = "A 39% hit rate"; + $x =~ s!(\d+)%!$1/100!e; # $x contains "A 0.39 hit rate" -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 -If the empty regexp C<//> is used, the string is split into individual -characters. If the regexp has groupings, then list produced contains + $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 list produced contains the matched substrings from the groupings as well: $x = "/usr/bin"; @@ -450,7 +455,7 @@ the matched substrings from the groupings as well: # $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 @@ -460,7 +465,7 @@ None. =head1 SEE ALSO This is just a quick start guide. For a more in-depth tutorial on -regexps, see L<perlretut> and for the reference page, see L<perlre>. +regexes, see L<perlretut> and for the reference page, see L<perlre>. =head1 AUTHOR AND COPYRIGHT @@ -469,5 +474,11 @@ 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 |