X-Git-Url: https://perl5.git.perl.org/perl5.git/blobdiff_plain/563642b4907d9b1b6beaa96b472ae787ae81d56f..fb5f378b17e3b41db03064c19b9205db64a3354c:/pod/perlretut.pod diff --git a/pod/perlretut.pod b/pod/perlretut.pod index d72d52d..1e1cdd4 100644 --- a/pod/perlretut.pod +++ b/pod/perlretut.pod @@ -23,30 +23,30 @@ characteristics. The string is most often some text, such as a line, sentence, web page, or even a whole book, but less commonly it could be some binary data as well. Suppose we want to determine if the text in variable, C<$var> contains -the sequence of characters C C C C C C C C +the sequence of characters S> (blanks added for legibility). We can write in Perl $var =~ m/mushroom/ The value of this expression will be TRUE if C<$var> contains that sequence of characters, and FALSE otherwise. The portion enclosed in -C<"E"> characters denotes the characteristic we are looking for. +C<'E'> characters denotes the characteristic we are looking for. We use the term I for it. The process of looking to see if the pattern occurs in the string is called I, and the C<"=~"> -operator along with the C<"m//"> tell Perl to try to match the pattern +operator along with the C tell Perl to try to match the pattern against the string. Note that the pattern is also a string, but a very special kind of one, as we will see. Patterns are in common use these days; examples are the patterns typed into a search engine to find web pages -and the patterns used to list files in a directory, e.g., C -or C. In Perl, the patterns described by regular expressions +and the patterns used to list files in a directory, I, "C" +or "C". In Perl, the patterns described by regular expressions are used not only to search strings, but to also extract desired parts of strings, and to do search and replace operations. Regular expressions have the undeserved reputation of being abstract and difficult to understand. This really stems simply because the notation used to express them tends to be terse and dense, and not -because of inherent complexity. We recommend using the C<"/x"> regular +because of inherent complexity. We recommend using the C regular expression modifier (described below) along with plenty of white space to make them less dense, and easier to read. Regular expressions are constructed using @@ -64,7 +64,7 @@ comfortable with the basics and hungry for more power tools. It discusses the more advanced regular expression operators and introduces the latest cutting-edge innovations. -A note: to save time, 'regular expression' is often abbreviated as +A note: to save time, "regular expression" is often abbreviated as regexp or regex. Regexp is a more natural abbreviation than regex, but is harder to pronounce. The Perl pod documentation is evenly split on regexp vs regex; in Perl, there is more than one way to abbreviate it. @@ -112,7 +112,7 @@ be reversed by using the C operator: The literal string in the regexp can be replaced by a variable: - $greeting = "World"; + my $greeting = "World"; if ("Hello World" =~ /$greeting/) { print "It matches\n"; } @@ -140,7 +140,7 @@ to arbitrary delimiters by putting an C<'m'> out front: # '/' becomes an ordinary char C, C, and C all represent the -same thing. When, e.g., the quote (C<">) is used as a delimiter, the forward +same thing. When, I, the quote (C<'"'>) is used as a delimiter, the forward slash C<'/'> becomes an ordinary character and can be used in this regexp without trouble. @@ -154,10 +154,10 @@ Let's consider how different regexps would match C<"Hello World">: The first regexp C doesn't match because regexps are case-sensitive. The second regexp matches because the substring S> occurs in the string S>. The space -character ' ' is treated like any other character in a regexp and is +character C<' '> is treated like any other character in a regexp and is needed to match in this case. The lack of a space character is the reason the third regexp C<'oW'> doesn't match. The fourth regexp -C<'World '> doesn't match because there is a space at the end of the +"C" doesn't match because there is a space at the end of the regexp, but not at the end of the string. The lesson here is that regexps must match a part of the string I in order for the statement to be true. @@ -169,15 +169,22 @@ always match at the earliest possible point in the string: "That hat is red" =~ /hat/; # matches 'hat' in 'That' With respect to character matching, there are a few more points you -need to know about. First of all, not all characters can be used 'as -is' in a match. Some characters, called I, are reserved -for use in regexp notation. The metacharacters are +need to know about. First of all, not all characters can be used "as +is" in a match. Some characters, called I, are +generally reserved for use in regexp notation. The metacharacters are - {}[]()^$.|*+?\ + {}[]()^$.|*+?-#\ + +This list is not as definitive as it may appear (or be claimed to be in +other documentation). For example, C<"#"> is a metacharacter only when +the C pattern modifier (described below) is used, and both C<"}"> +and C<"]"> are metacharacters only when paired with opening C<"{"> or +C<"["> respectively; other gotchas apply. The significance of each of these will be explained in the rest of the tutorial, but for now, it is important only to know -that a metacharacter can be matched by putting a backslash before it: +that a metacharacter can be matched as-is 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 + @@ -197,13 +204,21 @@ be backslashed: 'C:\WIN32' =~ /C:\\WIN/; # matches +In situations where it doesn't make sense for a particular metacharacter +to mean what it normally does, it automatically loses its +metacharacter-ness and becomes an ordinary character that is to be +matched literally. For example, the C<'}'> is a metacharacter only when +it is the mate of a C<'{'> metacharacter. Otherwise it is treated as a +literal RIGHT CURLY BRACKET. This may lead to unexpected results. +L|re/'strict' mode> can catch some of these. + In addition to the metacharacters, there are some ASCII characters which don't have printable character equivalents and are instead represented by I. Common examples are C<\t> for a tab, C<\n> for a newline, C<\r> for a carriage return and C<\a> for a bell (or alert). If your string is better thought of as a sequence of arbitrary -bytes, the octal escape sequence, e.g., C<\033>, or hexadecimal escape -sequence, e.g., C<\x1B> may be a more natural representation for your +bytes, the octal escape sequence, I, C<\033>, or hexadecimal escape +sequence, I, C<\x1B> may be a more natural representation for your bytes. Here are some examples of escapes: "1000\t2000" =~ m(0\t2) # matches @@ -262,9 +277,9 @@ C use the default variable C<$_> implicitly. With all of the regexps above, if the regexp matched anywhere in the string, it was considered a match. Sometimes, however, we'd like to specify I in the string the regexp should try to match. To do -this, we would use the I metacharacters C<^> and C<$>. The -anchor C<^> means match at the beginning of the string and the anchor -C<$> means match at the end of the string, or before a newline at the +this, we would use the I metacharacters C<'^'> and C<'$'>. The +anchor C<'^'> means match at the beginning of the string and the anchor +C<'$'> means match at the end of the string, or before a newline at the end of the string. Here is how they are used: "housekeeper" =~ /keeper/; # matches @@ -272,13 +287,13 @@ end of the string. Here is how they are used: "housekeeper" =~ /keeper$/; # matches "housekeeper\n" =~ /keeper$/; # matches -The second regexp doesn't match because C<^> constrains C to +The second regexp doesn't match because C<'^'> constrains C to match only at the beginning of the string, but C<"housekeeper"> has keeper starting in the middle. The third regexp does match, since the -C<$> constrains C to match only at the end of the string. +C<'$'> constrains C to match only at the end of the string. -When both C<^> and C<$> are used at the same time, the regexp has to -match both the beginning and the end of the string, i.e., the regexp +When both C<'^'> and C<'$'> are used at the same time, the regexp has to +match both the beginning and the end of the string, I, the regexp matches the whole string. Consider "keeper" =~ /^keep$/; # doesn't match @@ -287,7 +302,7 @@ matches the whole string. Consider The first regexp doesn't match because the string has more to it than C. Since the second regexp is exactly the string, it -matches. Using both C<^> and C<$> in a regexp forces the complete +matches. Using both C<'^'> and C<'$'> in a regexp forces the complete string to match, so it gives you complete control over which strings match and which don't. Suppose you are looking for a fellow named bert, off in a string by himself: @@ -343,13 +358,13 @@ operation. We will meet other modifiers later in the tutorial. We saw in the section above that there were ordinary characters, which represented themselves, and special characters, which needed a -backslash C<\> to represent themselves. The same is true in a +backslash C<'\'> to represent themselves. The same is true in a character class, 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<-]\^$> (and the pattern delimiter, whatever it is). -C<]> is special because it denotes the end of a character class. C<$> is -special because it denotes a scalar variable. C<\> is special because +C<']'> is special because it denotes the end of a character class. C<'$'> is +special because it denotes a scalar variable. C<'\'> is special because it is used in escape sequences, just like above. Here is how the special characters C<]$\> are handled: @@ -360,7 +375,7 @@ special characters C<]$\> are handled: /[\\$x]at/; # matches '\at', 'bat, 'cat', or 'rat' The last two are a little tricky. In C<[\$x]>, the backslash protects -the dollar sign, so the character class has two members C<$> and C. +the dollar sign, so the character class has two members C<'$'> and C<'x'>. In C<[\\$x]>, the backslash is protected, so C<$x> is treated as a variable and substituted in double quote fashion. @@ -380,7 +395,7 @@ If C<'-'> is the first or last character in a character class, it is treated as an ordinary character; C<[-ab]>, C<[ab-]> and C<[a\-b]> are all equivalent. -The special character C<^> in the first position of a character class +The special character C<'^'> in the first position of a character class denotes a I, which matches any character but those in the brackets. Both C<[...]> and C<[^...]> must match a character, or the match fails. Then @@ -393,7 +408,7 @@ character, or the match fails. Then Now, even C<[0-9]> can be a bother to write multiple times, so in the interest of saving keystrokes and making regexps more readable, Perl has several abbreviations for common character classes, as shown below. -Since the introduction of Unicode, unless the C modifier is in +Since the introduction of Unicode, unless the C modifier is in effect, these character classes match more than just a few characters in the ASCII range. @@ -401,46 +416,46 @@ the ASCII range. =item * -\d matches a digit, not just [0-9] but also digits from non-roman scripts +C<\d> matches a digit, not just C<[0-9]> but also digits from non-roman scripts =item * -\s matches a whitespace character, the set [\ \t\r\n\f] and others +C<\s> matches a whitespace character, the set C<[\ \t\r\n\f]> and others =item * -\w matches a word character (alphanumeric or _), not just [0-9a-zA-Z_] +C<\w> matches a word character (alphanumeric or C<'_'>), not just C<[0-9a-zA-Z_]> but also digits and characters from non-roman scripts =item * -\D is a negated \d; it represents any other character than a digit, or [^\d] +C<\D> is a negated C<\d>; it represents any other character than a digit, or C<[^\d]> =item * -\S is a negated \s; it represents any non-whitespace character [^\s] +C<\S> is a negated C<\s>; it represents any non-whitespace character C<[^\s]> =item * -\W is a negated \w; it represents any non-word character [^\w] +C<\W> is a negated C<\w>; it represents any non-word character C<[^\w]> =item * -The period '.' matches any character but "\n" (unless the modifier C is +The period C<'.'> matches any character but C<"\n"> (unless the modifier C is in effect, as explained below). =item * -\N, like the period, matches any character but "\n", but it does so -regardless of whether the modifier C is in effect. +C<\N>, like the period, matches any character but C<"\n">, but it does so +regardless of whether the modifier C is in effect. =back -The C modifier, available starting in Perl 5.14, is used to -restrict the matches of \d, \s, and \w to just those in the ASCII range. +The C modifier, available starting in Perl 5.14, is used to +restrict the matches of C<\d>, C<\s>, and C<\w> to just those in the ASCII range. It is useful to keep your program from being needlessly exposed to full Unicode (and its accompanying security considerations) when all you want -is to process English-like text. (The "a" may be doubled, C, to +is to process English-like text. (The "a" may be doubled, C, to provide even more restrictions, preventing case-insensitive matching of ASCII with non-ASCII characters; otherwise a Unicode "Kelvin Sign" would caselessly match a "k" or "K".) @@ -502,48 +517,48 @@ of it as empty. Then This behavior is convenient, because we usually want to ignore newlines when we count and match characters in a line. Sometimes, -however, we want to keep track of newlines. We might even want C<^> -and C<$> to anchor at the beginning and end of lines within the +however, we want to keep track of newlines. We might even want C<'^'> +and C<'$'> to anchor at the beginning and end of lines within the string, rather than just the beginning and end of the string. Perl allows us to choose between ignoring and paying attention to newlines -by using the C and C modifiers. C and C stand for +by using the C and C modifiers. C and C stand for single line and multi-line and they determine whether a string is to be treated as one continuous string, or as a set of lines. The two modifiers affect two aspects of how the regexp is interpreted: 1) how -the C<'.'> character class is defined, and 2) where the anchors C<^> -and C<$> are able to match. Here are the four possible combinations: +the C<'.'> character class is defined, and 2) where the anchors C<'^'> +and C<'$'> are able to match. Here are the four possible combinations: =over 4 =item * -no modifiers (//): Default behavior. C<'.'> matches any character -except C<"\n">. C<^> matches only at the beginning of the string and -C<$> matches only at the end or before a newline at the end. +no modifiers: Default behavior. C<'.'> matches any character +except C<"\n">. C<'^'> matches only at the beginning of the string and +C<'$'> matches only at the end or before a newline at the end. =item * -s modifier (//s): Treat string as a single long line. C<'.'> matches -any character, even C<"\n">. C<^> matches only at the beginning of -the string and C<$> matches only at the end or before a newline at the +s modifier (C): Treat string as a single long line. C<'.'> matches +any character, even C<"\n">. C<'^'> matches only at the beginning of +the string and C<'$'> matches only at the end or before a newline at the end. =item * -m modifier (//m): Treat string as a set of multiple lines. C<'.'> -matches any character except C<"\n">. C<^> and C<$> are able to match +m modifier (C): Treat string as a set of multiple lines. C<'.'> +matches any character except C<"\n">. C<'^'> and C<'$'> are able to match at the start or end of I line within the string. =item * -both s and m modifiers (//sm): Treat string as a single long line, but +both s and m modifiers (C): Treat string as a single long line, but detect multiple lines. C<'.'> matches any character, even -C<"\n">. C<^> and C<$>, however, are able to match at the start or end +C<"\n">. C<'^'> and C<'$'>, however, are able to match at the start or end of I line within the string. =back -Here are examples of C and C in action: +Here are examples of C and C in action: $x = "There once was a girl\nWho programmed in Perl\n"; @@ -557,11 +572,11 @@ Here are examples of C and C in action: $x =~ /girl.Who/m; # doesn't match, "." doesn't match "\n" $x =~ /girl.Who/sm; # matches, "." matches "\n" -Most of the time, the default behavior is what is wanted, but C and -C are occasionally very useful. If C is being used, the start +Most of the time, the default behavior is what is wanted, but C and +C are occasionally very useful. If C is being used, the start of the string can still be matched with C<\A> and the end of the string can still be matched with the anchors C<\Z> (matches both the end and -the newline before, like C<$>), and C<\z> (matches only the end): +the newline before, like C<'$'>), and C<\z> (matches only the end): $x =~ /^Who/m; # matches, "Who" at start of second line $x =~ /\AWho/m; # doesn't match, "Who" is not at start of string @@ -580,7 +595,7 @@ choices are described in the next section. Sometimes we would like our regexp to be able to match different possible words or character strings. This is accomplished by using -the I metacharacter C<|>. To match C or C, we +the I metacharacter C<'|'>. To match C or C, we form the regexp C. As before, Perl will try to match the regexp at the earliest possible point in the string. At each character position, Perl will first try to match the first @@ -654,7 +669,7 @@ C<"20"> is two digits. The process of trying one alternative, seeing if it matches, and moving on to the next alternative, while going back in the string from where the previous alternative was tried, if it doesn't, is called -I. The term 'backtracking' comes from the idea that +I. The term "backtracking" comes from the idea that matching a regexp is like a walk in the woods. Successfully matching a regexp is like arriving at a destination. There are many possible trailheads, one for each string position, and each one is tried in @@ -672,62 +687,59 @@ of what Perl does when it tries to match the regexp =over 4 -=item Z<>0 - -Start with the first letter in the string 'a'. +=item Z<>0. Start with the first letter in the string C<'a'>. -=item Z<>1 +E -Try the first alternative in the first group 'abd'. +=item Z<>1. Try the first alternative in the first group C<'abd'>. -=item Z<>2 +E -Match 'a' followed by 'b'. So far so good. +=item Z<>2. Match C<'a'> followed by C<'b'>. So far so good. -=item Z<>3 +E -'d' in the regexp doesn't match 'c' in the string - a dead -end. So backtrack two characters and pick the second alternative in -the first group 'abc'. +=item Z<>3. C<'d'> in the regexp doesn't match C<'c'> in the string - a +dead end. So backtrack two characters and pick the second alternative +in the first group C<'abc'>. -=item Z<>4 +E -Match 'a' followed by 'b' followed by 'c'. We are on a roll -and have satisfied the first group. Set $1 to 'abc'. +=item Z<>4. Match C<'a'> followed by C<'b'> followed by C<'c'>. We are on a roll +and have satisfied the first group. Set C<$1> to C<'abc'>. -=item Z<>5 +E -Move on to the second group and pick the first alternative -'df'. +=item Z<>5 Move on to the second group and pick the first alternative C<'df'>. -=item Z<>6 +E -Match the 'd'. +=item Z<>6 Match the C<'d'>. -=item Z<>7 +E -'f' in the regexp doesn't match 'e' in the string, so a dead +=item Z<>7. C<'f'> in the regexp doesn't match C<'e'> in the string, so a dead end. Backtrack one character and pick the second alternative in the -second group 'd'. +second group C<'d'>. -=item Z<>8 +E -'d' matches. The second grouping is satisfied, so set $2 to -'d'. +=item Z<>8. C<'d'> matches. The second grouping is satisfied, so set +C<$2> to C<'d'>. -=item Z<>9 +E -We are at the end of the regexp, so we are done! We have -matched 'abcd' out of the string "abcde". +=item Z<>9. We are at the end of the regexp, so we are done! We have +matched C<'abcd'> out of the string C<"abcde">. =back There are a couple of things to note about this analysis. First, the -third alternative in the second group 'de' also allows a match, but we +third alternative in the second group C<'de'> also allows a match, but we stopped before we got to it - at a given character position, leftmost wins. Second, we were able to get a match at the first character -position of the string 'a'. If there were no matches at the first -position, Perl would move to the second character position 'b' and +position of the string C<'a'>. If there were no matches at the first +position, Perl would move to the second character position C<'b'> and attempt the match all over again. Only when all possible paths at all possible character positions have been exhausted does Perl give up and declare S> to be false. @@ -744,7 +756,7 @@ The grouping metacharacters C<()> also serve another completely different function: they allow the extraction of the parts of a string that matched. This is very useful to find out what matched and for text processing in general. For each grouping, the part that matched -inside goes into the special variables C<$1>, C<$2>, etc. They can be +inside goes into the special variables C<$1>, C<$2>, I. They can be used just as ordinary variables: # extract hours, minutes, seconds @@ -764,7 +776,7 @@ C<($1,$2,$3)>. So we could write the code more compactly as If the groupings in a regexp are nested, C<$1> gets the group with the leftmost opening parenthesis, C<$2> the next opening parenthesis, -etc. Here is a regexp with nested groups: +I. Here is a regexp with nested groups: /(ab(cd|ef)((gi)|j))/; 1 2 34 @@ -776,7 +788,7 @@ or it remains undefined. For convenience, Perl sets C<$+> to the string held by the highest numbered C<$1>, C<$2>,... that got assigned (and, somewhat related, C<$^N> to the -value of the C<$1>, C<$2>,... most-recently assigned; i.e. the C<$1>, +value of the C<$1>, C<$2>,... most-recently assigned; I the C<$1>, C<$2>,... associated with the rightmost closing parenthesis used in the match). @@ -788,12 +800,12 @@ the I C<\g1>, C<\g2>,... Backreferences are simply matching variables that can be used I a regexp. This is a really nice feature; what matches later in a regexp is made to depend on what matched earlier in the regexp. Suppose we wanted to look -for doubled words in a text, like 'the the'. The following regexp finds +for doubled words in a text, like "the the". The following regexp finds all 3-letter doubles with a space in between: /\b(\w\w\w)\s\g1\b/; -The grouping assigns a value to \g1, so that the same 3-letter sequence +The grouping assigns a value to C<\g1>, so that the same 3-letter sequence is used for both parts. A similar task is to find words consisting of two identical parts: @@ -807,7 +819,7 @@ A similar task is to find words consisting of two identical parts: papa The regexp has a single grouping which considers 4-letter -combinations, then 3-letter combinations, etc., and uses C<\g1> to look for +combinations, then 3-letter combinations, I., and uses C<\g1> to look for a repeat. Although C<$1> and C<\g1> represent the same thing, care should be taken to use matched variables C<$1>, C<$2>,... only I a regexp and backreferences C<\g1>, C<\g2>,... only I a regexp; not doing @@ -861,14 +873,14 @@ capture group is accessible through the C<%+> hash. Assuming that we have to match calendar dates which may be given in one of the three formats yyyy-mm-dd, mm/dd/yyyy or dd.mm.yyyy, we can write -three suitable patterns where we use 'd', 'm' and 'y' respectively as the +three suitable patterns where we use C<'d'>, C<'m'> and C<'y'> respectively as the names of the groups capturing the pertaining components of a date. The matching operation combines the three patterns as alternatives: $fmt1 = '(?\d\d\d\d)-(?\d\d)-(?\d\d)'; $fmt2 = '(?\d\d)/(?\d\d)/(?\d\d\d\d)'; $fmt3 = '(?\d\d)\.(?\d\d)\.(?\d\d\d\d)'; - for my $d qw( 2006-10-21 15.01.2007 10/31/2005 ){ + for my $d (qw(2006-10-21 15.01.2007 10/31/2005)) { if ( $d =~ m{$fmt1|$fmt2|$fmt3} ){ print "day=$+{d} month=$+{m} year=$+{y}\n"; } @@ -927,7 +939,7 @@ prints Even if there are no groupings in a regexp, it is still possible to find out what exactly matched in a string. If you use them, Perl will set C<$`> to the part of the string before the match, will set C<$&> -to the part of the string that matched, and will set C<$'> to the part +to the part of the string that matched, and will set C<'$'> to the part of the string after the match. An example: $x = "the cat caught the mouse"; @@ -936,10 +948,10 @@ of the string after the match. An example: In the second match, C<$`> equals C<''> because the regexp matched at the first character position in the string and stopped; it never saw the -second 'the'. +second "the". If your code is to run on Perl versions earlier than -5.20, it is worthwhile to note that using C<$`> and C<$'> +5.20, it is worthwhile to note that using C<$`> and C<'$'> slows down regexp matching quite a bit, while C<$&> slows it down to a lesser extent, because if they are used in one regexp in a program, they are generated for I regexps in the program. So if raw @@ -956,7 +968,7 @@ variables may be used. These are only set if the C

modifier is present. Consequently they do not penalize the rest of the program. In Perl 5.20, C<${^PREMATCH}>, C<${^MATCH}> and C<${^POSTMATCH}> are available whether the C

has been used or not (the modifier is ignored), and -C<$`>, C<$'> and C<$&> do not cause any speed difference. +C<$`>, C<'$'> and C<$&> do not cause any speed difference. =head2 Non-capturing groupings @@ -1003,8 +1015,8 @@ less. We'd like to be able to match words or, more generally, strings of any length, without writing out tedious alternatives like C<\w\w\w\w|\w\w\w|\w\w|\w>. -This is exactly the problem the I metacharacters C, -C<*>, C<+>, and C<{}> were created for. They allow us to delimit the +This is exactly the problem the I metacharacters C<'?'>, +C<'*'>, C<'+'>, and C<{}> were created for. They allow us to delimit the number of repeats for a portion of a regexp 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 @@ -1014,15 +1026,15 @@ meanings: =item * -C means: match 'a' 1 or 0 times +C means: match C<'a'> 1 or 0 times =item * -C means: match 'a' 0 or more times, i.e., any number of times +C means: match C<'a'> 0 or more times, I, any number of times =item * -C means: match 'a' 1 or more times, i.e., at least once +C means: match C<'a'> 1 or more times, I, at least once =item * @@ -1062,9 +1074,9 @@ Here are some examples: For all of these quantifiers, Perl will try to match as much of the string as possible, while still allowing the regexp to succeed. Thus -with C, Perl will first try to match the regexp with the C +with C, Perl will first try to match the regexp with the C<'a'> present; if that fails, Perl will try to match the regexp without the -C present. For the quantifier C<*>, we get the following: +C<'a'> present. For the quantifier C<'*'>, we get the following: $x = "the cat in the hat"; $x =~ /^(.*)(cat)(.*)$/; # matches, @@ -1111,7 +1123,7 @@ that allows a match for the whole regexp will be the one used. =item * -Principle 2: The maximal matching quantifiers C, C<*>, C<+> and +Principle 2: The maximal matching quantifiers C<'?'>, C<'*'>, C<'+'> and C<{n,m}> will in general match as much of the string as possible while still allowing the whole regexp to match. @@ -1141,8 +1153,8 @@ Here is an example of these principles in action: # $3 = 'l' This regexp matches at the earliest string position, C<'T'>. One -might think that C, being leftmost in the alternation, would be -matched, but C produces the longest string in the first quantifier. +might think that C<'e'>, being leftmost in the alternation, would be +matched, but C<'r'> produces the longest string in the first quantifier. $x =~ /(m{1,2})(.*)$/; # matches, # $1 = 'mm' @@ -1167,7 +1179,7 @@ C<'m'> for the second quantifier C. Here, C<.?> eats its maximal one character at the earliest possible position in the string, C<'a'> in C, leaving C -the opportunity to match both C's. Finally, +the opportunity to match both C<'m'>'s. Finally, "aXXXb" =~ /(X*)/; # matches with $1 = '' @@ -1179,23 +1191,23 @@ Sometimes greed is not good. At times, we would like quantifiers to match a I piece of string, rather than a maximal piece. For this purpose, Larry Wall created the I or I quantifiers C, C<*?>, C<+?>, and C<{}?>. These are -the usual quantifiers with a C appended to them. They have the +the usual quantifiers with a C<'?'> appended to them. They have the following meanings: =over 4 =item * -C means: match 'a' 0 or 1 times. Try 0 first, then 1. +C means: match C<'a'> 0 or 1 times. Try 0 first, then 1. =item * -C means: match 'a' 0 or more times, i.e., any number of times, +C means: match C<'a'> 0 or more times, I, any number of times, but as few times as possible =item * -C means: match 'a' 1 or more times, i.e., at least once, but +C means: match C<'a'> 1 or more times, I, at least once, but as few times as possible =item * @@ -1224,9 +1236,9 @@ Let's look at the example above, but with minimal quantifiers: # $2 = 'e' # $3 = ' programming republic of Perl' -The minimal string that will allow both the start of the string C<^> +The minimal string that will allow both the start of the string C<'^'> and the alternation to match is C, with the alternation C -matching C. The second quantifier C<.*> is free to gobble up the +matching C<'e'>. The second quantifier C<.*> is free to gobble up the rest of the string. $x =~ /(m{1,2}?)(.*?)$/; # matches, @@ -1237,7 +1249,7 @@ The first string position that this regexp can match is at the first C<'m'> in C. At this position, the minimal C matches just one C<'m'>. Although the second quantifier C<.*?> would prefer to match no characters, it is constrained by the end-of-string -anchor C<$> to match the rest of the string. +anchor C<'$'> to match the rest of the string. $x =~ /(.*?)(m{1,2}?)(.*)$/; # matches, # $1 = 'The progra' @@ -1245,12 +1257,12 @@ anchor C<$> to match the rest of the string. # $3 = 'ming republic of Perl' In this regexp, you might expect the first minimal quantifier C<.*?> -to match the empty string, because it is not constrained by a C<^> +to match the empty string, because it is not constrained by a C<'^'> anchor to match the beginning of the word. Principle 0 applies here, however. Because it is possible for the whole regexp to match at the start of the string, it I match at the start of the string. Thus -the first quantifier has to match everything up to the first C. The -second minimal quantifier matches just one C and the third +the first quantifier has to match everything up to the first C<'m'>. The +second minimal quantifier matches just one C<'m'> and the third quantifier matches the rest of the string. $x =~ /(.??)(m{1,2})(.*)$/; # matches, @@ -1291,37 +1303,36 @@ backtracking. Here is a step-by-step analysis of the example =over 4 -=item Z<>0 +=item Z<>0. Start with the first letter in the string C<'t'>. -Start with the first letter in the string 't'. +E -=item Z<>1 +=item Z<>1. The first quantifier C<'.*'> starts out by matching the whole +string "C". -The first quantifier '.*' starts out by matching the whole -string 'the cat in the hat'. +E -=item Z<>2 +=item Z<>2. C<'a'> in the regexp element C<'at'> doesn't match the end +of the string. Backtrack one character. -'a' in the regexp element 'at' doesn't match the end of the -string. Backtrack one character. +E -=item Z<>3 +=item Z<>3. C<'a'> in the regexp element C<'at'> still doesn't match +the last letter of the string C<'t'>, so backtrack one more character. -'a' in the regexp element 'at' still doesn't match the last -letter of the string 't', so backtrack one more character. +E -=item Z<>4 +=item Z<>4. Now we can match the C<'a'> and the C<'t'>. -Now we can match the 'a' and the 't'. +E -=item Z<>5 - -Move on to the third element '.*'. Since we are at the end of -the string and '.*' can match 0 times, assign it the empty string. +=item Z<>5. Move on to the third element C<'.*'>. Since we are at the +end of the string and C<'.*'> can match 0 times, assign it the empty +string. -=item Z<>6 +E -We are done! +=item Z<>6. We are done! =back @@ -1333,14 +1344,14 @@ string. A typical structure that blows up in your face is of the form /(a|b+)*/; The problem is the nested indeterminate quantifiers. There are many -different ways of partitioning a string of length n between the C<+> -and C<*>: one repetition with C of length n, two repetitions with +different ways of partitioning a string of length n between the C<'+'> +and C<'*'>: one repetition with C of length n, two repetitions with the first C length k and the second with length n-k, m repetitions -whose bits add up to length n, etc. In fact there are an exponential +whose bits add up to length n, I. In fact there are an exponential number of ways to partition a string as a function of its length. A regexp may get lucky and match early in the process, but if there is no match, Perl will try I possibility before giving up. So be -careful with nested C<*>'s, C<{n,m}>'s, and C<+>'s. The book +careful with nested C<'*'>'s, C<{n,m}>'s, and C<'+'>'s. The book I by Jeffrey Friedl gives a wonderful discussion of this and other efficiency issues. @@ -1355,15 +1366,15 @@ the simple pattern Whenever this is applied to a string which doesn't quite meet the pattern's expectations such as S> or S>, -the regex engine will backtrack, approximately once for each character +the regexp engine will backtrack, approximately once for each character in the string. But we know that there is no way around taking I of the initial word characters to match the first repetition, that I spaces must be eaten by the middle part, and the same goes for the second word. With the introduction of the I in Perl 5.10, we -have a way of instructing the regex engine not to backtrack, with the -usual quantifiers with a C<+> appended to them. This makes them greedy as +have a way of instructing the regexp engine not to backtrack, with the +usual quantifiers with a C<'+'> appended to them. This makes them greedy as well as stingy; once they succeed they won't give anything back to permit another solution. They have the following meanings: @@ -1451,12 +1462,12 @@ Now consider floating point numbers with exponents. The key observation here is that I integers and numbers with decimal points are allowed in front of an exponent. Then exponents, like the overall sign, are independent of whether we are matching numbers with -or without decimal points, and can be 'decoupled' from the +or without decimal points, and can be "decoupled" from the mantissa. The overall form of the regexp now becomes clear: /^(optional sign)(integer | f.p. mantissa)(optional exponent)$/; -The exponent is an C or C, followed by an integer. So the +The exponent is an C<'e'> or C<'E'>, followed by an integer. So the exponent regexp is /[eE][+-]?\d+/; # exponent @@ -1466,10 +1477,10 @@ Putting all the parts together, we get a regexp that matches numbers: /^[+-]?(\d+\.\d+|\d+\.|\.\d+|\d+)([eE][+-]?\d+)?$/; # Ta da! Long regexps like this may impress your friends, but can be hard to -decipher. In complex situations like this, the C modifier for a +decipher. In complex situations like this, the C modifier for a match is invaluable. It allows one to put nearly arbitrary whitespace and comments into a regexp without affecting their meaning. Using it, -we can rewrite our 'extended' regexp in the more pleasing form +we can rewrite our "extended" regexp in the more pleasing form /^ [+-]? # first, match an optional sign @@ -1516,7 +1527,25 @@ could be factored out: ( [eE] [+-]? \d+ )? # finally, optionally match an exponent $/x; -or written in the compact form, +Starting in Perl v5.26, specifying C changes the square-bracketed +portions of a pattern to ignore tabs and space characters unless they +are escaped by preceding them with a backslash. So, we could write + + /^ + [ + - ]?\ * # first, match an optional sign + ( # then match integers or f.p. mantissas: + \d+ # start out with a ... + ( + \.\d* # mantissa of the form a.b or a. + )? # ? takes care of integers of the form a + |\.\d+ # mantissa of the form .b + ) + ( [ e E ] [ + - ]? \d+ )? # finally, optionally match an exponent + $/xx; + +This doesn't really improve the legibility of this example, but it's +available in case you want it. Squashing the pattern down to the +compact form, we have /^[+-]?\ *(\d+(\.\d*)?|\.\d+)([eE][+-]?\d+)?$/; @@ -1560,8 +1589,8 @@ We have already introduced the matching operator in its default C and arbitrary delimiter C forms. We have used the binding operator C<=~> and its negation C to test for string matches. Associated with the matching operator, we have discussed the -single line C, multi-line C, case-insensitive C and -extended C modifiers. There are a few more things you might +single line C, multi-line C, case-insensitive C and +extended C modifiers. There are a few more things you might want to know about matching operators. =head3 Prohibiting substitution @@ -1576,7 +1605,7 @@ special delimiter C: } Similar to strings, C acts like apostrophes on a regexp; all other -C delimiters act like quotes. If the regexp evaluates to the empty string, +C<'m'> delimiters act like quotes. If the regexp evaluates to the empty string, the regexp in the I is used instead. So we have "dog" =~ /d/; # 'd' matches @@ -1586,15 +1615,15 @@ the regexp in the I is used instead. So we have =head3 Global matching The final two modifiers we will discuss here, -C and C, concern multiple matches. -The modifier C stands for global matching and allows the +C and C, concern multiple matches. +The modifier C stands for global matching and allows the matching operator to match within a string as many times as possible. In scalar context, successive invocations against a string will have -C jump from match to match, keeping track of position in the +C 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 function. -The use of C is shown in the following example. Suppose we have +The use of C is shown in the following example. Suppose we have a string that consists of words separated by spaces. If we know how many words there are in advance, we could extract the words using groupings: @@ -1606,7 +1635,7 @@ groupings: # $3 = 'house' But what if we had an indeterminate number of words? This is the sort -of task C was made for. To extract all words, form the simple +of task C was made for. To extract all words, form the simple regexp C<(\w+)> and loop over all matches with C: while ($x =~ /(\w+)/g) { @@ -1621,12 +1650,12 @@ 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, as in C. The current position in the string is +C, as in C. The current position in the string is associated with the string, not the regexp. This means that different strings have different positions and their respective positions can be set or read independently. -In list context, C returns a list of matched groupings, or if +In list context, C returns a list of matched groupings, or if there are no groupings, a list of matches to the whole regexp. So if we wanted just the words, we could use @@ -1635,8 +1664,8 @@ we wanted just the words, we could use # $words[1] = 'dog' # $words[2] = 'house' -Closely associated with the C modifier is the C<\G> anchor. The -C<\G> anchor matches at the point where the previous C match left +Closely associated with the C modifier is the C<\G> anchor. The +C<\G> anchor matches at the point where the previous C match left off. C<\G> allows us to easily do context-sensitive matching: $metric = 1; # use metric units @@ -1652,7 +1681,7 @@ off. C<\G> allows us to easily do context-sensitive matching: } $x =~ /\G\s+(widget|sprocket)/g; # continue processing -The combination of C and C<\G> allows us to process the string a +The combination of C and C<\G> allows us to process the string a bit at a time and use arbitrary Perl logic to decide what to do next. Currently, the C<\G> anchor is only fully supported when used to anchor to the start of the pattern. @@ -1669,7 +1698,7 @@ naive regexp $dna =~ /TGA/; doesn't work; it may match a C, but there is no guarantee that -the match is aligned with codon boundaries, e.g., the substring +the match is aligned with codon boundaries, I, the substring S> gives a match. A better solution is while ($dna =~ /(\w\w\w)*?TGA/g) { # note the minimal *? @@ -1702,7 +1731,7 @@ important not only to match what is desired, but to reject what is not desired. (There are other regexp modifiers that are available, such as -C, but their specialized uses are beyond the +C, but their specialized uses are beyond the scope of this introduction. ) =head3 Search and replace @@ -1712,7 +1741,7 @@ operations in Perl. Search and replace is accomplished with the C operator. The general form is C, with everything we know about regexps and modifiers applying in this case as well. The -C is a Perl double-quoted string that replaces in the +I is a Perl double-quoted string that replaces in the string whatever is matched with the C. The operator C<=~> is also used here to associate a string with C. If matching against C<$_>, the S> can be dropped. If there is a match, @@ -1730,7 +1759,7 @@ false. Here are a few examples: In the last example, the whole string was matched, but only the part inside the single quotes was grouped. With the C operator, the -matched variables C<$1>, C<$2>, etc. are immediately available for use +matched variables C<$1>, C<$2>, I. are immediately available for use in the replacement expression, so we use C<$1> to replace the quoted string with just what was quoted. With the global modifier, C will search and replace all occurrences of the regexp in the string: @@ -1742,7 +1771,7 @@ will search and replace all occurrences of the regexp in the string: $x =~ s/4/four/g; # does it all: # $x contains "I batted four for four" -If you prefer 'regex' over 'regexp' in this tutorial, you could use +If you prefer "regex" over "regexp" in this tutorial, you could use the following program to replace it: % cat > simple_replace @@ -1820,7 +1849,7 @@ such as C and C, and even C. If single quotes are used C, then the regexp and replacement are treated as single-quoted strings and there are no variable substitutions. C in list context -returns the same thing as in scalar context, i.e., the number of +returns the same thing as in scalar context, I, the number of matches. =head3 The split function @@ -1855,7 +1884,7 @@ groupings as well. For instance, # $parts[5] = '/' # $parts[6] = 'perl' -Since the first character of $x matched the regexp, C prepended +Since the first character of C<$x> matched the regexp, C prepended an empty initial element to the list. If you have read this far, congratulations! You now have all the basic @@ -1914,7 +1943,7 @@ instance, $x = "\QThat !^*&%~& cat!"; $x =~ /\Q!^*&%~&\E/; # check for rough language -It does not protect C<$> or C<@>, so that variables can still be +It does not protect C<'$'> or C<'@'>, so that variables can still be substituted. C<\Q>, C<\L>, C<\l>, C<\U>, C<\u> and C<\E> are actually part of @@ -1936,8 +1965,9 @@ to know 1) how to represent Unicode characters in a regexp and 2) that a matching operation will treat the string to be searched as a sequence of characters, not bytes. The answer to 1) is that Unicode characters greater than C are represented using the C<\x{hex}> notation, because -\x hex (without curly braces) doesn't go further than 255. (Starting in Perl -5.14, if you're an octal fan, you can also use C<\o{oct}>.) +C<\x>I (without curly braces and I are two hex digits) doesn't +go further than 255. (Starting in Perl 5.14, if you're an octal fan, +you can also use C<\o{oct}>.) /\x{263a}/; # match a Unicode smiley face :) @@ -1977,7 +2007,7 @@ L. The answer to requirement 2) is that a regexp (mostly) uses Unicode characters. The "mostly" is for messy backward -compatibility reasons, but starting in Perl 5.14, any regex compiled in +compatibility reasons, but starting in Perl 5.14, any regexp compiled in the scope of a C (which is automatically turned on within the scope of a C or higher) will turn that "mostly" into "always". If you want to handle Unicode properly, you @@ -1988,10 +2018,9 @@ it is a sequence of characters, not bytes. See L for a tutorial about that. Let us now discuss Unicode character classes, most usually called -"character properties". These are represented by the -C<\p{name}> escape sequence. Closely associated is the C<\P{name}> -property, which is the negation of the C<\p{name}> one. For -example, to match lower and uppercase characters, +"character properties". These are represented by the C<\p{I}> +escape sequence. The negation of this is C<\P{I}>. For example, +to match lower and uppercase characters, $x = "BOB"; $x =~ /^\p{IsUpper}/; # matches, uppercase char class @@ -1999,7 +2028,7 @@ example, to match lower and uppercase characters, $x =~ /^\p{IsLower}/; # doesn't match, lowercase char class $x =~ /^\P{IsLower}/; # matches, char class sans lowercase -(The "Is" is optional.) +(The "C" is optional.) There are many, many Unicode character properties. For the full list see L. Most of them have synonyms with shorter names, @@ -2015,14 +2044,14 @@ should generally use C.) For example, English, French, and a bunch of other European languages are written in the Latin script. But there is also the Greek script, the Thai script, -the Katakana script, etc. You can test whether a character is in a +the Katakana script, I. You can test whether a character is in a particular script (based on C) with, for example C<\p{Latin}>, C<\p{Greek}>, or C<\p{Katakana}>. To test if it isn't in the Balinese script, you would use C<\P{Balinese}>. What we have described so far is the single form of the C<\p{...}> character classes. There is also a compound form which you may run into. These -look like C<\p{name=value}> or C<\p{name:value}> (the equals sign and colon +look like C<\p{I=I}> or C<\p{I:I}> (the equals sign and colon can be used interchangeably). These are more general than the single form, and in fact most of the single forms are just Perl-defined shortcuts for common compound forms. For example, the script examples in the previous paragraph @@ -2035,28 +2064,28 @@ use can make your code easier to understand. C<\X> is an abbreviation for a character class that comprises a Unicode I. This represents a "logical character": what appears to be a single character, but may be represented internally by more -than one. As an example, using the Unicode full names, e.g., S> is a grapheme cluster with base character C and combining character -S>, which translates in Danish to A with the circle atop it, +than one. As an example, using the Unicode full names, I, "S" is a grapheme cluster with base character "A" and combining character +"S, which translates in Danish to "A" with the circle atop it, as in the word Engstrom. For the full and latest information about Unicode see the latest Unicode standard, or the Unicode Consortium's website L As if all those classes weren't enough, Perl also defines POSIX-style -character classes. These have the form C<[:name:]>, with C the +character classes. These have the form C<[:I:]>, with I the name of the POSIX class. The POSIX classes are C, C, C, C, C, C, C, C, C, C, C, and C, and two extensions, C (a Perl -extension to match C<\w>), and C (a GNU extension). The C +extension to match C<\w>), and C (a GNU extension). The C modifier restricts these to matching just in the ASCII range; otherwise they can match the same as their corresponding Perl Unicode classes: -C<[:upper:]> is the same as C<\p{IsUpper}>, etc. (There are some +C<[:upper:]> is the same as C<\p{IsUpper}>, I. (There are some exceptions and gotchas with this; see L for a full discussion.) The C<[:digit:]>, C<[:word:]>, and C<[:space:]> correspond to the familiar C<\d>, C<\w>, and C<\s> -character classes. To negate a POSIX class, put a C<^> in front of -the name, so that, e.g., C<[:^digit:]> corresponds to C<\D> and, under +character classes. To negate a POSIX class, put a C<'^'> in front of +the name, so that, I, C<[:^digit:]> corresponds to C<\D> and, under Unicode, C<\P{IsDigit}>. The Unicode and POSIX character classes can be used just like C<\d>, with the exception that POSIX character classes can only be used inside of a character class: @@ -2092,7 +2121,7 @@ C<$reg> can also be interpolated into a larger regexp: $x =~ /(abc)?$reg/; # still matches As with the matching operator, the regexp quote can use different -delimiters, e.g., C, C or C. Apostrophes +delimiters, I, C, C or C. Apostrophes as delimiters (C) inhibit any interpolation. Pre-compiled regexps are useful for creating dynamic matches that @@ -2214,9 +2243,9 @@ example is /(?# Match an integer:)[+-]?\d+/; This style of commenting has been largely superseded by the raw, -freeform commenting that is allowed with the C modifier. +freeform commenting that is allowed with the C modifier. -Most modifiers, such as C, C, C and C (or any +Most modifiers, such as C, C, C and C (or any combination thereof) can also be embedded in a regexp using C<(?i)>, C<(?m)>, C<(?s)>, and C<(?x)>. For instance, @@ -2229,7 +2258,7 @@ a regexp using C<(?i)>, C<(?m)>, C<(?s)>, and C<(?x)>. For instance, /x; Embedded modifiers can have two important advantages over the usual -modifiers. Embedded modifiers allow a custom set of modifiers to +modifiers. Embedded modifiers allow a custom set of modifiers for I regexp pattern. This is great for matching an array of regexps that must have different modifiers: @@ -2242,7 +2271,7 @@ that must have different modifiers: } } -The second advantage is that embedded modifiers (except C, which +The second advantage is that embedded modifiers (except C

, which modifies the entire regexp) only affect the regexp inside the group the embedded modifier is contained in. So grouping can be used to localize the modifier's effects: @@ -2250,8 +2279,8 @@ can be used to localize the modifier's effects: /Answer: ((?i)yes)/; # matches 'Answer: yes', 'Answer: YES', etc. Embedded modifiers can also turn off any modifiers already present -by using, e.g., C<(?-i)>. Modifiers can also be combined into -a single expression, e.g., C<(?s-i)> turns on single line mode and +by using, I, C<(?-i)>. Modifiers can also be combined into +a single expression, I, C<(?s-i)> turns on single line mode and turns off case insensitivity. Embedded modifiers may also be added to a non-capturing grouping. @@ -2264,13 +2293,13 @@ case insensitively and turns off multi-line mode. This section concerns the lookahead and lookbehind assertions. First, a little background. -In Perl regular expressions, most regexp elements 'eat up' a certain +In Perl regular expressions, most regexp elements "eat up" a certain amount of string when they match. For instance, the regexp element C<[abc]> eats up one character of the string when it matches, in the sense that Perl moves to the next character position in the string after the match. There are some elements, however, that don't eat up characters (advance the character position) if they match. The examples -we have seen so far are the anchors. The anchor C<^> matches the +we have seen so far are the anchors. The anchor C<'^'> matches the beginning of the line, but doesn't eat any characters. Similarly, the word boundary anchor C<\b> matches wherever a character matching C<\w> is next to a character that doesn't, but it doesn't eat up any @@ -2284,8 +2313,8 @@ checks out, we can proceed forward. But if the local environment doesn't satisfy us, we must backtrack. Checking the environment entails either looking ahead on the trail, -looking behind, or both. C<^> looks behind, to see that there are no -characters before. C<$> looks ahead, to see that there are no +looking behind, or both. C<'^'> looks behind, to see that there are no +characters before. C<'$'> looks ahead, to see that there are no characters after. C<\b> looks both ahead and behind, to see if the characters on either side differ in their "word-ness". @@ -2309,7 +2338,7 @@ non-capturing, since these are zero-width assertions. Thus in the second regexp, the substrings captured are those of the whole regexp itself. Lookahead C<(?=regexp)> can match arbitrary regexps, but lookbehind C<< (?<=fixed-regexp) >> only works for regexps of fixed -width, i.e., a fixed number of characters long. Thus +width, I, a fixed number of characters long. Thus C<< (?<=(ab|bc)) >> is fine, but C<< (?<=(ab)*) >> is not. The negated versions of the lookahead and lookbehind assertions are denoted by C<(?!regexp)> and C<< (?> respectively. @@ -2332,6 +2361,18 @@ by looking ahead and behind: | (?<=-) (?=\S) # a '-' followed by any non-space /x, $str; # @toks = qw(one two - - - 6 - 8) +Starting in Perl 5.28, experimentally, alphabetic equivalents to these +assertions are added, so you can use whichever is most memorable for +your tastes. + + (?=...) (*pla:...) or (*positive_lookahead:...) + (?!...) (*nla:...) or (*negative_lookahead:...) + (?<=...) (*plb:...) or (*positive_lookbehind:...) + (?...) (*atomic:...) + +Using any of these will raise (unless turned off) a warning in the +C category. =head2 Using independent subexpressions to prevent backtracking @@ -2347,9 +2388,9 @@ considering an ordinary regexp: $x =~ /a*ab/; # matches This obviously matches, but in the process of matching, the -subexpression C first grabbed the C. Doing so, however, +subexpression C first grabbed the C<'a'>. Doing so, however, wouldn't allow the whole regexp to match, so after backtracking, C -eventually gave back the C and matched the empty string. Here, what +eventually gave back the C<'a'> and matched the empty string. Here, what C matched was I on what the rest of the regexp matched. Contrast that with an independent subexpression: @@ -2357,17 +2398,17 @@ Contrast that with an independent subexpression: $x =~ /(?>a*)ab/; # doesn't match! The independent subexpression C<< (?>a*) >> doesn't care about the rest -of the regexp, so it sees an C and grabs it. Then the rest of the +of the regexp, so it sees an C<'a'> and grabs it. Then the rest of the regexp C cannot match. Because C<< (?>a*) >> is independent, there is no backtracking and the independent subexpression does not give -up its C. Thus the match of the regexp as a whole fails. A similar +up its C<'a'>. Thus the match of the regexp as a whole fails. A similar behavior occurs with completely independent regexps: $x = "ab"; $x =~ /a*/g; # matches, eats an 'a' $x =~ /\Gab/g; # doesn't match, no 'a' available -Here C and C<\G> create a 'tag team' handoff of the string from +Here C and C<\G> create a "tag team" handoff of the string from one regexp to the other. Regexps with an independent subexpression are much like this, with a handoff of the string to the independent subexpression, and a handoff of the string back to the enclosing @@ -2379,7 +2420,7 @@ enclosed in parentheses up to two levels deep. Then the following regexp matches: $x = "abc(de(fg)h"; # unbalanced parentheses - $x =~ /\( ( [^()]+ | \([^()]*\) )+ \)/x; + $x =~ /\( ( [ ^ () ]+ | \( [ ^ () ]* \) )+ \)/xx; The regexp matches an open parenthesis, one or more copies of an alternation, and a close parenthesis. The alternation is two-way, with @@ -2393,7 +2434,7 @@ was no match possible. To prevent the exponential blowup, we need to prevent useless backtracking at some point. This can be done by enclosing the inner quantifier as an independent subexpression: - $x =~ /\( ( (?>[^()]+) | \([^()]*\) )+ \)/x; + $x =~ /\( ( (?> [ ^ () ]+ ) | \([ ^ () ]* \) )+ \)/xx; Here, C<< (?>[^()]+) >> breaks the degeneracy of string partitioning by gobbling up as much of the string as possible and keeping it. Then @@ -2405,26 +2446,27 @@ match failures fail much more quickly. A I is a form of if-then-else statement that allows one to choose which patterns are to be matched, based on some condition. There are two types of conditional expression: -C<(?(condition)yes-regexp)> and -C<(?(condition)yes-regexp|no-regexp)>. C<(?(condition)yes-regexp)> is -like an S> statement in Perl. If the C is true, -the C will be matched. If the C is false, the -C will be skipped and Perl will move onto the next regexp +C<(?(I)I)> and +C<(?(condition)I|I)>. +C<(?(I)I)> is +like an S> statement in Perl. If the I is true, +the I will be matched. If the I is false, the +I will be skipped and Perl will move onto the next regexp element. The second form is like an S> statement -in Perl. If the C is true, the C will be -matched, otherwise the C will be matched. +in Perl. If the I is true, the I will be +matched, otherwise the I will be matched. -The C can have several forms. The first form is simply an -integer in parentheses C<(integer)>. It is true if the corresponding -backreference C<\integer> matched earlier in the regexp. The same +The I can have several forms. The first form is simply an +integer in parentheses C<(I)>. It is true if the corresponding +backreference C<\I> matched earlier in the regexp. The same thing can be done with a name associated with a capture group, written -as C<< () >> or C<< ('name') >>. The second form is a bare +as C<<< (EIE) >>> or C<< ('I') >>. The second form is a bare zero-width assertion C<(?...)>, either a lookahead, a lookbehind, or a code assertion (discussed in the next section). The third set of forms provides tests that return true if the expression is executed within a recursion (C<(R)>) or is being called from some capturing group, referenced either by number (C<(R1)>, C<(R2)>,...) or by name -(C<(R&name)>). +(C<(R&I)>). The integer or name form of the C allows us to choose, with more flexibility, what to match based on what matched earlier in the @@ -2447,7 +2489,7 @@ match. For instance, /[ATGC]+(?(?<=AA)G|C)$/; matches a DNA sequence such that it either ends in C, or some -other base pair combination and C. Note that the form is +other base pair combination and C<'C'>. Note that the form is C<< (?(?<=AA)G|C) >> and not C<< (?((?<=AA))G|C) >>; for the lookahead, lookbehind or code assertions, the parentheses around the conditional are not needed. @@ -2459,13 +2501,13 @@ Some regular expressions use identical subpatterns in several places. Starting with Perl 5.10, it is possible to define named subpatterns in a section of the pattern so that they can be called up by name anywhere in the pattern. This syntactic pattern for this definition -group is C<< (?(DEFINE)(?pattern)...) >>. An insertion -of a named pattern is written as C<(?&name)>. +group is C<< (?(DEFINE)(?>I)...) >>. An insertion +of a named pattern is written as C<(?&I)>. The example below illustrates this feature using the pattern for floating point numbers that was presented earlier on. The three subpatterns that are used more than once are the optional sign, the -digit sequence for an integer and the decimal fraction. The DEFINE +digit sequence for an integer and the decimal fraction. The C group at the end of the pattern contains their definition. Notice that the decimal fraction pattern is the first place where we can reuse the integer pattern. @@ -2485,7 +2527,7 @@ reuse the integer pattern. This feature (introduced in Perl 5.10) significantly extends the power of Perl's pattern matching. By referring to some other capture group anywhere in the pattern with the construct -C<(?group-ref)>, the I within the referenced group is used +C<(?I)>, the I within the referenced group is used as an independent subpattern in place of the group reference itself. Because the group reference may be contained I the group it refers to, it is now possible to apply pattern matching to tasks that @@ -2511,7 +2553,7 @@ have the full pattern: In C<(?...)> both absolute and relative backreferences may be used. The entire pattern can be reinserted with C<(?R)> or C<(?0)>. -If you prefer to name your groups, you can use C<(?&name)> to +If you prefer to name your groups, you can use C<(?&I)> to recurse into that group. @@ -2520,17 +2562,14 @@ recurse into that group. Normally, regexps are a part of Perl expressions. I expressions turn that around by allowing arbitrary Perl code to be a part of a regexp. A code evaluation -expression is denoted C<(?{code})>, with I a string of Perl +expression is denoted C<(?{I})>, with I a string of Perl statements. -Be warned that this feature is considered experimental, and may be -changed without notice. - Code expressions are zero-width assertions, and the value they return depends on their environment. There are two possibilities: either the code expression is used as a conditional in a conditional expression -C<(?(condition)...)>, or it is not. If the code expression is a -conditional, the code is evaluated and the result (i.e., the result of +C<(?(I)...)>, or it is not. If the code expression is a +conditional, the code is evaluated and the result (I, the result of the last statement) is used to determine truth or falsehood. If the code expression is not used as a conditional, the assertion always evaluates true and the result is put into the special variable @@ -2558,12 +2597,12 @@ example: Hmm. What happened here? If you've been following along, you know that the above pattern should be effectively (almost) the same as the last one; -enclosing the C in a character class isn't going to change what it +enclosing the C<'d'> in a character class isn't going to change what it matches. So why does the first not print while the second one does? -The answer lies in the optimizations the regex engine makes. In the first +The answer lies in the optimizations the regexp engine makes. In the first case, all the engine sees are plain old characters (aside from the -C construct). It's smart enough to realize that the string 'ddd' +C construct). It's smart enough to realize that the string C<'ddd'> doesn't occur in our target string before actually running the pattern through. But in the second case, we've tricked it into thinking that our pattern is more complicated. It takes a look, sees our @@ -2589,7 +2628,7 @@ backtracks in the process of searching for a match. If the regexp backtracks over a code expression and if the variables used within are localized using C, the changes in the variables produced by the code expression are undone! Thus, if we wanted to count how many times -a character got matched inside a group, we could use, e.g., +a character got matched inside a group, we could use, I, $x = "aaaa"; $count = 0; # initialize 'a' count @@ -2630,7 +2669,8 @@ The result C<$^R> is automatically localized, so that it will behave properly in the presence of backtracking. This example uses a code expression in a conditional to match a -definite article, either 'the' in English or 'der|die|das' in German: +definite article, either C<'the'> in English or C<'der|die|das'> in +German: $lang = 'DE'; # use German ... @@ -2644,8 +2684,8 @@ definite article, either 'the' in English or 'der|die|das' in German: ) /xi; -Note that the syntax here is C<(?(?{...})yes-regexp|no-regexp)>, not -C<(?((?{...}))yes-regexp|no-regexp)>. In other words, in the case of a +Note that the syntax here is C<(?(?{...})I|I)>, not +C<(?((?{...}))I|I)>. In other words, in the case of a code expression, we don't need the extra parentheses around the conditional. @@ -2703,7 +2743,7 @@ expression and matched immediately. A simple example is This final example contains both ordinary and pattern code expressions. It detects whether a binary string C<1101010010001...> has a -Fibonacci spacing 0,1,1,2,3,5,... of the C<1>'s: +Fibonacci spacing 0,1,1,2,3,5,... of the C<'1'>'s: $x = "1101010010001000001"; $z0 = ''; $z1 = '0'; # initial conditions @@ -2732,7 +2772,7 @@ expression. Rather, the whole code block is parsed as perl code at the same time as perl is compiling the code containing the literal regexp pattern. -The regexp without the C modifier is +This regexp without the C modifier is /^1(?:((??{ $z0 }))1(?{ $z0 = $z1; $z1 .= $^N; }))+$/ @@ -2745,11 +2785,9 @@ regexps is almost necessary in creating and debugging regexps. Perl 5.10 introduced a number of control verbs intended to provide detailed control over the backtracking process, by directly influencing -the regexp engine and by providing monitoring techniques. As all -the features in this group are experimental and subject to change or -removal in a future version of Perl, the interested reader is -referred to L for a -detailed description. +the regexp engine and by providing monitoring techniques. See +L for a detailed +description. Below is just one example, illustrating the control verb C<(*FAIL)>, which may be abbreviated as C<(*F)>. If this is inserted in a regexp @@ -2863,7 +2901,7 @@ part describes the compilation stage. C means that there is a starred object, in this case C<'a'>, and if it matches, goto line 4, -i.e., C. The middle lines describe some heuristics and +I, C. The middle lines describe some heuristics and optimizations performed before a match: floating 'bc' at 0..2147483647 (checking floating) minlen 2 @@ -2921,11 +2959,6 @@ prints t2 Done at position 4 -=head1 BUGS - -Code expressions, conditional expressions, and independent expressions -are I. Don't use them in production code. Yet. - =head1 SEE ALSO This is just a tutorial. For the full story on Perl regular @@ -2941,8 +2974,9 @@ Jeffrey Friedl (published by O'Reilly, ISBN 1556592-257-3). =head1 AUTHOR AND COPYRIGHT -Copyright (c) 2000 Mark Kvale +Copyright (c) 2000 Mark Kvale. All rights reserved. +Now maintained by Perl porters. This document may be distributed under the same terms as Perl itself.