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 @@ -1011,8 +1009,8 @@ 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 @@ -1022,15 +1020,15 @@ 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 * @@ -1070,9 +1068,9 @@ 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, @@ -1119,7 +1117,7 @@ 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. @@ -1149,8 +1147,8 @@ 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' @@ -1175,7 +1173,7 @@ 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 = '' @@ -1187,23 +1185,23 @@ 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 * @@ -1232,9 +1230,9 @@ 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, @@ -1245,7 +1243,7 @@ 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' @@ -1253,12 +1251,12 @@ 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, @@ -1299,37 +1297,36 @@ backtracking. Here is a step-by-step analysis of the example =over 4 -=item Z<>0 - -Start with the first letter in the string 't'. +=item Z<>0. Start with the first letter in the string C<'t'>. -=item Z<>1 +E -The first quantifier '.*' starts out by matching the whole -string 'the cat in the hat'. +=item Z<>1. The first quantifier C<'.*'> starts out by matching the whole +string "C". -=item Z<>2 +E -'a' in the regexp element 'at' doesn't match the end of the -string. Backtrack one character. +=item Z<>2. C<'a'> in the regexp element C<'at'> doesn't match the end +of the string. Backtrack one character. -=item Z<>3 +E -'a' in the regexp element 'at' still doesn't match the last -letter of the string 't', so backtrack one more character. +=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. -=item Z<>4 +E -Now we can match the 'a' and the 't'. +=item Z<>4. Now we can match the C<'a'> and the C<'t'>. -=item Z<>5 +E -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 @@ -1341,14 +1338,14 @@ 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. @@ -1363,15 +1360,15 @@ 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: @@ -1459,12 +1456,12 @@ 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 @@ -1477,7 +1474,7 @@ Long regexps like this may impress your friends, but can be hard to 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 @@ -1602,7 +1599,7 @@ 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 @@ -1695,7 +1692,7 @@ 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 *? @@ -1738,7 +1735,7 @@ 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, @@ -1756,7 +1753,7 @@ 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: @@ -1768,7 +1765,7 @@ 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 @@ -1846,7 +1843,7 @@ 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 @@ -1881,7 +1878,7 @@ 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 @@ -1940,7 +1937,7 @@ 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 @@ -1962,8 +1959,9 @@ 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 :) @@ -2003,7 +2001,7 @@ 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 @@ -2014,10 +2012,9 @@ 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 @@ -2025,7 +2022,7 @@ 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, @@ -2041,14 +2038,14 @@ 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 @@ -2061,28 +2058,28 @@ 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 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: @@ -2118,7 +2115,7 @@ 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 @@ -2255,7 +2252,7 @@ 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: @@ -2276,8 +2273,8 @@ 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. @@ -2290,13 +2287,13 @@ 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 @@ -2310,8 +2307,8 @@ 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". @@ -2335,7 +2332,7 @@ 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. @@ -2373,9 +2370,9 @@ 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: @@ -2383,17 +2380,17 @@ 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 @@ -2431,26 +2428,27 @@ 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 @@ -2473,7 +2471,7 @@ 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. @@ -2485,13 +2483,13 @@ 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. @@ -2511,7 +2509,7 @@ 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 @@ -2537,7 +2535,7 @@ 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. @@ -2546,17 +2544,14 @@ 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 @@ -2584,12 +2579,12 @@ 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 @@ -2615,7 +2610,7 @@ 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 @@ -2656,7 +2651,8 @@ 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 ... @@ -2670,8 +2666,8 @@ 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. @@ -2729,7 +2725,7 @@ 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 @@ -2758,7 +2754,7 @@ 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; }))+\$/ @@ -2771,11 +2767,9 @@ 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 @@ -2889,7 +2883,7 @@ 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 @@ -2947,11 +2941,6 @@ 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 @@ -2967,8 +2956,9 @@ 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. -- 1.8.3.1 ```