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Diffstat (limited to 'pod/perlre.pod')
| -rw-r--r-- | pod/perlre.pod | 190 |
1 files changed, 101 insertions, 89 deletions
diff --git a/pod/perlre.pod b/pod/perlre.pod index da32f873fc..927d088edb 100644 --- a/pod/perlre.pod +++ b/pod/perlre.pod @@ -10,7 +10,7 @@ operations, plus various examples of the same, see C<m//> and C<s///> in L<perlop>. The matching operations can have various modifiers. The modifiers -which relate to the interpretation of the regular expression inside +that relate to the interpretation of the regular expression inside are listed below. For the modifiers that alter the behaviour of the operation, see L<perlop/"m//"> and L<perlop/"s//">. @@ -34,8 +34,8 @@ line anywhere within the string, Treat string as single line. That is, change "." to match any character whatsoever, even a newline, which it normally would not match. -The /s and /m modifiers both override the C<$*> setting. That is, no matter -what C<$*> contains, /s (without /m) will force "^" to match only at the +The C</s> and C</m> modifiers both override the C<$*> setting. That is, no matter +what C<$*> contains, C</s> without C</m> will force "^" to match only at the beginning of the string and "$" to match only at the end (or just before a newline at the end) of the string. Together, as /ms, they let the "." match any character whatsoever, while yet allowing "^" and "$" to match, @@ -58,18 +58,19 @@ backslashed nor within a character class. You can use this to break up your regular expression into (slightly) more readable parts. The C<#> character is also treated as a metacharacter introducing a comment, just as in ordinary Perl code. This also means that if you want real -whitespace or C<#> characters in the pattern that you'll have to either +whitespace or C<#> characters in the pattern (outside of a character +class, where they are unaffected by C</x>), that you'll either have to escape them or encode them using octal or hex escapes. Taken together, these features go a long way towards making Perl's regular expressions more readable. Note that you have to be careful not to include the pattern delimiter in the comment--perl has no way of knowing you did -not intend to close the pattern early. See the C comment deletion code +not intend to close the pattern early. See the C-comment deletion code in L<perlop>. =head2 Regular Expressions The patterns used in pattern matching are regular expressions such as -those supplied in the Version 8 regexp routines. (In fact, the +those supplied in the Version 8 regex routines. (In fact, the routines are derived (distantly) from Henry Spencer's freely redistributable reimplementation of the V8 routines.) See L<Version 8 Regular Expressions> for details. @@ -146,7 +147,7 @@ also work: \L lowercase till \E (think vi) \U uppercase till \E (think vi) \E end case modification (think vi) - \Q quote (disable) regexp metacharacters till \E + \Q quote (disable) pattern metacharacters till \E If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u> and C<\U> is taken from the current locale. See L<perllocale>. @@ -165,7 +166,7 @@ In addition, Perl defines the following: \d Match a digit character \D Match a non-digit character -Note that C<\w> matches a single alphanumeric character, not a whole +A C<\w> matches a single alphanumeric character, not a whole word. To match a word you'd need to say C<\w+>. If C<use locale> is in effect, the list of alphabetic characters generated by C<\w> is taken from the current locale. See L<perllocale>. You may use C<\w>, C<\W>, @@ -185,7 +186,7 @@ has a C<\w> on one side of it and a C<\W> on the other side of it (in either order), counting the imaginary characters off the beginning and end of the string as matching a C<\W>. (Within character classes C<\b> represents backspace rather than a word boundary.) The C<\A> and C<\Z> are -just like "^" and "$" except that they won't match multiple times when the +just like "^" and "$", except that they won't match multiple times when the C</m> modifier is used, while "^" and "$" will match at every internal line boundary. To match the actual end of the string, not ignoring newline, you can use C<\Z(?!\n)>. The C<\G> assertion can be used to chain global @@ -193,7 +194,7 @@ matches (using C<m//g>), as described in L<perlop/"Regexp Quote-Like Operators">. It is also useful when writing C<lex>-like scanners, when you have several -regexps which you want to match against consequent substrings of your +patterns that you want to match against consequent substrings of your string, see the previous reference. The actual location where C<\G> will match can also be influenced by using C<pos()> as an lvalue. See L<perlfunc/pos>. @@ -233,21 +234,21 @@ Once perl sees that you need one of C<$&>, C<$`> or C<$'> anywhere in the program, it has to provide them on each and every pattern match. This can slow your program down. The same mechanism that handles these provides for the use of $1, $2, etc., so you pay the same price -for each regexp that contains capturing parentheses. But if you never -use $&, etc., in your script, then regexps I<without> capturing +for each pattern that contains capturing parentheses. But if you never +use $&, etc., in your script, then patterns I<without> capturing parentheses won't be penalized. So avoid $&, $', and $` if you can, but if you can't (and some algorithms really appreciate them), once you've used them once, use them at will, because you've already paid -the price. +the price. As of 5.005, $& is not so costly as the other two. -You will note that all backslashed metacharacters in Perl are +Backslashed metacharacters in Perl are alphanumeric, such as C<\b>, C<\w>, C<\n>. Unlike some other regular expression languages, there are no backslashed symbols that aren't alphanumeric. So anything that looks like \\, \(, \), \E<lt>, \E<gt>, \{, or \} is always interpreted as a literal character, not a metacharacter. This was once used in a common idiom to disable or quote the special meanings of regular expression metacharacters in a -string that you want to use for a pattern. Simply quote all the +string that you want to use for a pattern. Simply quote all non-alphanumeric characters: $pattern =~ s/(\W)/\\$1/g; @@ -271,31 +272,32 @@ function of the extension. Several extensions are already supported: A comment. The text is ignored. If the C</x> switch is used to enable whitespace formatting, a simple C<#> will suffice. -=item C<(?:regexp)> +=item C<(?:pattern)> -This groups things like "()" but doesn't make backreferences like "()" does. So +This is for clustering, not capturing; it groups subexpressions like +"()", but doesn't make backreferences as "()" does. So - split(/\b(?:a|b|c)\b/) + @fields = split(/\b(?:a|b|c)\b/) is like - split(/\b(a|b|c)\b/) + @fields = split(/\b(a|b|c)\b/) but doesn't spit out extra fields. -=item C<(?=regexp)> +=item C<(?=pattern)> A zero-width positive lookahead assertion. For example, C</\w+(?=\t)/> matches a word followed by a tab, without including the tab in C<$&>. -=item C<(?!regexp)> +=item C<(?!pattern)> A zero-width negative lookahead assertion. For example C</foo(?!bar)/> matches any occurrence of "foo" that isn't followed by "bar". Note however that lookahead and lookbehind are NOT the same thing. You cannot use this for lookbehind. -If you are looking for a "bar" which isn't preceded by a "foo", C</(?!foo)bar/> +If you are looking for a "bar" that isn't preceded by a "foo", C</(?!foo)bar/> will not do what you want. That's because the C<(?!foo)> is just saying that the next thing cannot be "foo"--and it's not, it's a "bar", so "foobar" will match. You would have to do something like C</(?!foo)...bar/> for that. We @@ -307,13 +309,13 @@ Sometimes it's still easier just to say: For lookbehind see below. -=item C<(?<=regexp)> +=item C<(?E<lt>=pattern)> -A zero-width positive lookbehind assertion. For example, C</(?=\t)\w+/> +A zero-width positive lookbehind assertion. For example, C</(?E<lt>=\t)\w+/> matches a word following a tab, without including the tab in C<$&>. Works only for fixed-width lookbehind. -=item C<(?<!regexp)> +=item C<(?<!pattern)> A zero-width negative lookbehind assertion. For example C</(?<!bar)foo/> matches any occurrence of "foo" that isn't following "bar". @@ -325,70 +327,80 @@ Experimental "evaluate any Perl code" zero-width assertion. Always succeeds. C<code> is not interpolated. Currently the rules to determine where the C<code> ends are somewhat convoluted. -=item C<(?E<gt>regexp)> +B<WARNING>: This is a grave security risk for arbitrarily interpolated +patterns. It introduces security holes in previously safe programs. +A fix to Perl, and to this documentation, will be forthcoming prior +to the actual 5.005 release. -An "independend" subexpression. Matches the substring which a -I<standalone> C<regexp> would match if anchored at the given position, +=item C<(?E<gt>pattern)> + +An "independent" subexpression. Matches the substring that a +I<standalone> C<pattern> would match if anchored at the given position, B<and only this substring>. Say, C<^(?E<gt>a*)ab> will never match, since C<(?E<gt>a*)> (anchored -at the beginning of string, as above) will match I<all> the characters +at the beginning of string, as above) will match I<all> characters C<a> at the beginning of string, leaving no C<a> for C<ab> to match. In contrast, C<a*ab> will match the same as C<a+b>, since the match of the subgroup C<a*> is influenced by the following group C<ab> (see L<"Backtracking">). In particular, C<a*> inside C<a*ab> will match less characters that a standalone C<a*>, since this makes the tail match. -Note that a similar effect to C<(?E<gt>regexp)> may be achieved by +An effect similar to C<(?E<gt>pattern)> may be achieved by - (?=(regexp))\1 + (?=(pattern))\1 since the lookahead is in I<"logical"> context, thus matches the same substring as a standalone C<a+>. The following C<\1> eats the matched string, thus making a zero-length assertion into an analogue of -C<(?>...)>. (The difference of these two constructions is that the -second one uses a catching group, thus shifts ordinals of +C<(?>...)>. (The difference between these two constructs is that the +second one uses a catching group, thus shifting ordinals of backreferences in the rest of a regular expression.) -This construction is very useful for optimizations of "eternal" -matches, since it will not backtrack (see L<"Backtracking">). Say, +This construct is useful for optimizations of "eternal" +matches, because it will not backtrack (see L<"Backtracking">). - / \( ( + m{ \( ( [^()]+ | \( [^()]* \) )+ - \) /x - -will match a nonempty group with matching two-or-less-level-deep -parentheses. It is very efficient in finding such groups. However, -if there is no such group, it is going to take forever (on reasonably -long string), since there are so many different ways to split a long -string into several substrings (this is essentially what C<(.+)+> is -doing, and this is a subpattern of the above pattern). Say, on -C<((()aaaaaaaaaaaaaaaaaa> the above pattern detects no-match in 5sec -(on kitchentop'96 processor), and each extra letter doubles this time. - -However, a tiny modification of this - - / \( ( + \) + }x + +That will efficiently match a nonempty group with matching +two-or-less-level-deep parentheses. However, if there is no such group, +it will take virtually forever on a long string. That's because there are +so many different ways to split a long string into several substrings. +This is essentially what C<(.+)+> is doing, and this is a subpattern +of the above pattern. Consider that C<((()aaaaaaaaaaaaaaaaaa> on the +pattern above detects no-match in several seconds, but that each extra +letter doubles this time. This exponential performance will make it +appear that your program has hung. + +However, a tiny modification of this pattern + + m{ \( ( (?> [^()]+ ) | \( [^()]* \) )+ - \) /x + \) + }x -which uses (?>...) matches exactly when the above one does (it is a -good excercise to check this), but finishes in a fourth of the above -time on a similar string with 1000000 C<a>s. +which uses C<(?E<gt>...)> matches exactly when the one above does (verifying +this yourself would be a productive exercise), but finishes in a fourth +the time when used on a similar string with 1000000 C<a>s. Be aware, +however, that this pattern currently triggers a warning message under +B<-w> saying it C<"matches the null string many times">): -Note that on simple groups like the above C<(?> [^()]+ )> a similar +On simple groups, such as the pattern C<(?> [^()]+ )>, a comparable effect may be achieved by negative lookahead, as in C<[^()]+ (?! [^()] )>. This was only 4 times slower on a string with 1000000 C<a>s. -=item C<(?(condition)yes-regexp|no-regexp)> +=item C<(?(condition)yes-pattern|no-pattern)> -=item C<(?(condition)yes-regexp)> +=item C<(?(condition)yes-pattern)> Conditional expression. C<(condition)> should be either an integer in parentheses (which is valid if the corresponding pair of parentheses @@ -396,14 +408,15 @@ matched), or lookahead/lookbehind/evaluate zero-width assertion. Say, - / ( \( )? + m{ ( \( )? [^()]+ - (?(1) \) )/x + (?(1) \) ) + }x matches a chunk of non-parentheses, possibly included in parentheses themselves. -=item C<(?imstx)> +=item C<(?imsx)> One or more embedded pattern-match modifiers. This is particularly useful for patterns that are specified in a table somewhere, some of @@ -412,15 +425,14 @@ insensitive ones need to include merely C<(?i)> at the front of the pattern. For example: $pattern = "foobar"; - if ( /$pattern/i ) + if ( /$pattern/i ) { } # more flexible: $pattern = "(?i)foobar"; - if ( /$pattern/ ) + if ( /$pattern/ ) { } -Note that these modifiers are localized inside an enclosing group (if -any). Say, +These modifiers are localized inside an enclosing group (if any). Say, ( (?i) blah ) \s+ \1 @@ -430,15 +442,15 @@ case. =back -The specific choice of question mark for this and the new minimal -matching construct was because 1) question mark is pretty rare in older -regular expressions, and 2) whenever you see one, you should stop -and "question" exactly what is going on. That's psychology... +A question mark was chosen for this and for the new minimal-matching +construct because 1) question mark is pretty rare in older regular +expressions, and 2) whenever you see one, you should stop and "question" +exactly what is going on. That's psychology... =head2 Backtracking A fundamental feature of regular expression matching involves the -notion called I<backtracking>. which is currently used (when needed) +notion called I<backtracking>, which is currently used (when needed) by all regular expression quantifiers, namely C<*>, C<*?>, C<+>, C<+?>, C<{n,m}>, and C<{n,m}?>. @@ -539,7 +551,8 @@ As you see, this can be a bit tricky. It's important to realize that a regular expression is merely a set of assertions that gives a definition of success. There may be 0, 1, or several different ways that the definition might succeed against a particular string. And if there are -multiple ways it might succeed, you need to understand backtracking to know which variety of success you will achieve. +multiple ways it might succeed, you need to understand backtracking to +know which variety of success you will achieve. When using lookahead assertions and negations, this can all get even tricker. Imagine you'd like to find a sequence of non-digits not @@ -578,14 +591,13 @@ non-digits, you have something that's not 123?" If the pattern matcher had let C<\D*> expand to "ABC", this would have caused the whole pattern to fail. The search engine will initially match C<\D*> with "ABC". Then it will -try to match C<(?!123> with "123" which, of course, fails. But because +try to match C<(?!123> with "123", which of course fails. But because a quantifier (C<\D*>) has been used in the regular expression, the search engine can backtrack and retry the match differently in the hope of matching the complete regular expression. -Well now, -the pattern really, I<really> wants to succeed, so it uses the -standard regexp back-off-and-retry and lets C<\D*> expand to just "AB" this +The pattern really, I<really> wants to succeed, so it uses the +standard pattern back-off-and-retry and lets C<\D*> expand to just "AB" this time. Now there's indeed something following "AB" that is not "123". It's in fact "C123", which suffices. @@ -601,7 +613,7 @@ you'd expect; that is, case 5 will fail, but case 6 succeeds: 6: got ABC -In other words, the two zero-width assertions next to each other work like +In other words, the two zero-width assertions next to each other work as though they're ANDed together, just as you'd use any builtin assertions: C</^$/> matches only if you're at the beginning of the line AND the end of the line simultaneously. The deeper underlying truth is that juxtaposition in @@ -621,31 +633,31 @@ And if you used C<*>'s instead of limiting it to 0 through 5 matches, then it would take literally forever--or until you ran out of stack space. A powerful tool for optimizing such beasts is "independent" groups, -which do not backtrace (see L<C<(?E<gt>regexp)>>). Note also that +which do not backtrace (see L<C<(?E<gt>pattern)>>). Note also that zero-length lookahead/lookbehind assertions will not backtrace to make the tail match, since they are in "logical" context: only the fact whether they match or not is considered relevant. For an example where side-effects of a lookahead I<might> have influenced the -following match, see L<C<(?E<gt>regexp)>>. +following match, see L<C<(?E<gt>pattern)>>. =head2 Version 8 Regular Expressions -In case you're not familiar with the "regular" Version 8 regexp +In case you're not familiar with the "regular" Version 8 regex routines, here are the pattern-matching rules not described above. Any single character matches itself, unless it is a I<metacharacter> with a special meaning described here or above. You can cause -characters which normally function as metacharacters to be interpreted +characters that normally function as metacharacters to be interpreted literally by prefixing them with a "\" (e.g., "\." matches a ".", not any character; "\\" matches a "\"). A series of characters matches that series of characters in the target string, so the pattern C<blurfl> would match "blurfl" in the target string. You can specify a character class, by enclosing a list of characters -in C<[]>, which will match any one of the characters in the list. If the +in C<[]>, which will match any one character from the list. If the first character after the "[" is "^", the class matches any character not in the list. Within a list, the "-" character is used to specify a -range, so that C<a-z> represents all the characters between "a" and "z", +range, so that C<a-z> represents all characters between "a" and "z", inclusive. If you want "-" itself to be a member of a class, put it at the start or end of the list, or escape it with a backslash. (The following all specify the same class of three characters: C<[-az]>, @@ -663,7 +675,7 @@ character except "\n" (unless you use C</s>). You can specify a series of alternatives for a pattern using "|" to separate them, so that C<fee|fie|foe> will match any of "fee", "fie", -or "foe" in the target string (as would C<f(e|i|o)e>). Note that the +or "foe" in the target string (as would C<f(e|i|o)e>). The first alternative includes everything from the last pattern delimiter ("(", "[", or the beginning of the pattern) up to the first "|", and the last alternative contains everything from the last "|" to the next @@ -671,7 +683,7 @@ pattern delimiter. For this reason, it's common practice to include alternatives in parentheses, to minimize confusion about where they start and end. -Note that alternatives are tried from left to right, so the first +Alternatives are tried from left to right, so the first alternative found for which the entire expression matches, is the one that is chosen. This means that alternatives are not necessarily greedy. For example: when mathing C<foo|foot> against "barefoot", only the "foo" @@ -679,23 +691,23 @@ part will match, as that is the first alternative tried, and it successfully matches the target string. (This might not seem important, but it is important when you are capturing matched text using parentheses.) -Also note that "|" is interpreted as a literal within square brackets, +Also remember that "|" is interpreted as a literal within square brackets, so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>. Within a pattern, you may designate subpatterns for later reference by enclosing them in parentheses, and you may refer back to the I<n>th subpattern later in the pattern using the metacharacter \I<n>. Subpatterns are numbered based on the left to right order of their -opening parenthesis. Note that a backreference matches whatever +opening parenthesis. A backreference matches whatever actually matched the subpattern in the string being examined, not the rules for that subpattern. Therefore, C<(0|0x)\d*\s\1\d*> will -match "0x1234 0x4321",but not "0x1234 01234", because subpattern 1 +match "0x1234 0x4321", but not "0x1234 01234", because subpattern 1 actually matched "0x", even though the rule C<0|0x> could potentially match the leading 0 in the second number. =head2 WARNING on \1 vs $1 -Some people get too used to writing things like +Some people get too used to writing things like: $pattern =~ s/(\W)/\\\1/g; @@ -707,7 +719,7 @@ meaning of C<\1> is kludged in for C<s///>. However, if you get into the habit of doing that, you get yourself into trouble if you then add an C</e> modifier. - s/(\d+)/ \1 + 1 /eg; + s/(\d+)/ \1 + 1 /eg; # causes warning under -w Or if you try to do @@ -726,4 +738,4 @@ L<perlfunc/pos>. L<perllocale>. -"Mastering Regular Expressions" (see L<perlbook>) by Jeffrey Friedl. +I<Mastering Regular Expressions> (see L<perlbook>) by Jeffrey Friedl. |