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Diffstat (limited to 'pod/perlre.pod')
| -rw-r--r-- | pod/perlre.pod | 211 |
1 files changed, 199 insertions, 12 deletions
diff --git a/pod/perlre.pod b/pod/perlre.pod index 014ee3c818..1c7855c041 100644 --- a/pod/perlre.pod +++ b/pod/perlre.pod @@ -16,7 +16,7 @@ the regular expression inside. These are: i Do case-insensitive pattern matching. m Treat string as multiple lines. s Treat string as single line. - x Extend your pattern's legibilty with whitespace and comments. + x Extend your pattern's legibility with whitespace and comments. These are usually written as "the C</x> modifier", even though the delimiter in question might not actually be a slash. In fact, any of these @@ -46,7 +46,7 @@ meanings: \ Quote the next metacharacter ^ Match the beginning of the line . Match any character (except newline) - $ Match the end of the line + $ Match the end of the line (or before newline at the end) | Alternation () Grouping [] Character class @@ -80,7 +80,7 @@ The following standard quantifiers are recognized: (If a curly bracket occurs in any other context, it is treated as a regular character.) The "*" modifier is equivalent to C<{0,}>, the "+" modifier to C<{1,}>, and the "?" modifier to C<{0,1}>. n and m are limited -to integral values less than 65536. +to integral values less than 65536. By default, a quantified subpattern is "greedy", that is, it will match as many times as possible without causing the rest pattern not to match. The @@ -136,7 +136,7 @@ Perl defines the following zero-width assertions: \b Match a word boundary \B Match a non-(word boundary) \A Match only at beginning of string - \Z Match only at end of string + \Z Match only at end of string (or before newline at the end) \G Match only where previous m//g left off A word boundary (C<\b>) is defined as a spot between two characters that @@ -146,7 +146,8 @@ 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 "$" except that they won't match multiple times when the C</m> modifier is used, while "^" and "$" will match at every internal line -boundary. +boundary. To match the actual end of the string, not ignoring newline, +you can use C<\Z(?!\n)>. When the bracketing construct C<( ... )> is used, \<digit> matches the digit'th substring. Outside of the pattern, always use "$" instead of "\" @@ -162,7 +163,7 @@ You may have as many parentheses as you wish. If you have more than 9 substrings, the variables $10, $11, ... refer to the corresponding substring. Within the pattern, \10, \11, etc. refer back to substrings if there have been at least that many left parens before -the backreference. Otherwise (for backward compatibilty) \10 is the +the backreference. Otherwise (for backward compatibility) \10 is the same as \010, a backspace, and \11 the same as \011, a tab. And so on. (\1 through \9 are always backreferences.) @@ -192,7 +193,7 @@ non-alphanumeric characters: You can also use the built-in quotemeta() function to do this. An even easier way to quote metacharacters right in the match operator -is to say +is to say /$unquoted\Q$quoted\E$unquoted/ @@ -237,10 +238,10 @@ 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 say "like" because there's the case of your "bar" not having three characters before it. You could -cover that this way: C</(?:(?!foo)...|^..?)bar/>. Sometimes it's still +cover that this way: C</(?:(?!foo)...|^..?)bar/>. Sometimes it's still easier just to say: - if (/foo/ && $` =~ /bar$/) + if (/foo/ && $` =~ /bar$/) =item (?imsx) @@ -252,12 +253,12 @@ insensitive ones merely need to include 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/ ) =back @@ -266,6 +267,192 @@ 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... +=head2 Backtracking + +A fundamental feature of regular expression matching involves the notion +called I<backtracking>. which is used (when needed) by all regular +expression quantifiers, namely C<*>, C<*?>, C<+>, C<+?>, C<{n,m}>, and +C<{n,m}?>. + +For a regular expression to match, the I<entire> regular expression must +match, not just part of it. So if the beginning of a pattern containing a +quantifier succeeds in a way that causes later parts in the pattern to +fail, the matching engine backs up and recalculates the beginning +part--that's why it's called backtracking. + +Here is an example of backtracking: Let's say you want to find the +word following "foo" in the string "Food is on the foo table.": + + $_ = "Food is on the foo table."; + if ( /\b(foo)\s+(\w+)/i ) { + print "$2 follows $1.\n"; + } + +When the match runs, the first part of the regular expression (C<\b(foo)>) +finds a possible match right at the beginning of the string, and loads up +$1 with "Foo". However, as soon as the matching engine sees that there's +no whitespace following the "Foo" that it had saved in $1, it realizes its +mistake and starts over again one character after where it had had the +tentative match. This time it goes all the way until the next occurrence +of "foo". The complete regular expression matches this time, and you get +the expected output of "table follows foo." + +Sometimes minimal matching can help a lot. Imagine you'd like to match +everything between "foo" and "bar". Initially, you write something +like this: + + $_ = "The food is under the bar in the barn."; + if ( /foo(.*)bar/ ) { + print "got <$1>\n"; + } + +Which perhaps unexpectedly yields: + + got <d is under the bar in the > + +That's because C<.*> was greedy, so you get everything between the +I<first> "foo" and the I<last> "bar". In this case, it's more effective +to use minimal matching to make sure you get the text between a "foo" +and the first "bar" thereafter. + + if ( /foo(.*?)bar/ ) { print "got <$1>\n" } + got <d is under the > + +Here's another example: let's say you'd like to match a number at the end +of a string, and you also want to keep the preceding part the match. +So you write this: + + $_ = "I have 2 numbers: 53147"; + if ( /(.*)(\d*)/ ) { # Wrong! + print "Beginning is <$1>, number is <$2>.\n"; + } + +That won't work at all, because C<.*> was greedy and gobbled up the +whole string. As C<\d*> can match on an empty string the complete +regular expression matched successfully. + + Beginning is <I have 2: 53147>, number is <>. + +Here are some variants, most of which don't work: + + $_ = "I have 2 numbers: 53147"; + @pats = qw{ + (.*)(\d*) + (.*)(\d+) + (.*?)(\d*) + (.*?)(\d+) + (.*)(\d+)$ + (.*?)(\d+)$ + (.*)\b(\d+)$ + (.*\D)(\d+)$ + }; + + for $pat (@pats) { + printf "%-12s ", $pat; + if ( /$pat/ ) { + print "<$1> <$2>\n"; + } else { + print "FAIL\n"; + } + } + +That will print out: + + (.*)(\d*) <I have 2 numbers: 53147> <> + (.*)(\d+) <I have 2 numbers: 5314> <7> + (.*?)(\d*) <> <> + (.*?)(\d+) <I have > <2> + (.*)(\d+)$ <I have 2 numbers: 5314> <7> + (.*?)(\d+)$ <I have 2 numbers: > <53147> + (.*)\b(\d+)$ <I have 2 numbers: > <53147> + (.*\D)(\d+)$ <I have 2 numbers: > <53147> + +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 in +order 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 nondigits not +followed by "123". You might try to write that as + + $_ = "ABC123"; + if ( /^\D*(?!123)/ ) { # Wrong! + print "Yup, no 123 in $_\n"; + } + +But that isn't going to match; at least, not the way you're hoping. It +claims that there is no 123 in the string. Here's a clearer picture of +why it that pattern matches, contrary to popular expectations: + + $x = 'ABC123' ; + $y = 'ABC445' ; + + print "1: got $1\n" if $x =~ /^(ABC)(?!123)/ ; + print "2: got $1\n" if $y =~ /^(ABC)(?!123)/ ; + + print "3: got $1\n" if $x =~ /^(\D*)(?!123)/ ; + print "4: got $1\n" if $y =~ /^(\D*)(?!123)/ ; + +This prints + + 2: got ABC + 3: got AB + 4: got ABC + +You might have expected test 3 to fail because it just seems to a more +general purpose version of test 1. The important difference between +them is that test 3 contains a quantifier (C<\D*>) and so can use +backtracking, whereas test 1 will not. What's happening is +that you've asked "Is it true that at the start of $x, following 0 or more +nondigits, 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 +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 backoff-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. + +We can deal with this by using both an assertion and a negation. We'll +say that the first part in $1 must be followed by a digit, and in fact, it +must also be followed by something that's not "123". Remember that the +lookaheads are zero-width expressions--they only look, but don't consume +any of the string in their match. So rewriting this way produces what +you'd expect; that is, case 5 will fail, but case 6 succeeds: + + print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/ ; + print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/ ; + + 6: got ABC + +In other words, the two zero-width assertions next to each other work like +they're ANDed together, just as you'd use any builtin assertions: C</^$/> +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 +regular expressions always means AND, except when you write an explicit OR +using the vertical bar. C</ab/> means match "a" AND (then) match "b", +although the attempted matches are made at different positions because "a" +is not a zero-width assertion, but a one-width assertion. + +One warning: particularly complicated regular expressions can take +exponential time to solve due to the immense number of possible ways they +can use backtracking to try match. For example this will take a very long +time to run + + /((a{0,5}){0,5}){0,5}/ + +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. + =head2 Version 8 Regular Expressions In case you're not familiar with the "regular" Version 8 regexp @@ -309,7 +496,7 @@ 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>. +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 actually matched the subpattern in the string being examined, not the |