diff options
Diffstat (limited to 'pod/perlre.pod')
| -rw-r--r-- | pod/perlre.pod | 23 |
1 files changed, 16 insertions, 7 deletions
diff --git a/pod/perlre.pod b/pod/perlre.pod index 373e1ca84e..e985377586 100644 --- a/pod/perlre.pod +++ b/pod/perlre.pod @@ -278,16 +278,16 @@ matches a word followed by a tab, without including the tab in C<$&>. 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: C</(?!foo)bar/> will not find an occurrence of -"bar" that is preceded by something which is not "foo". That's because +use this for lookbehind. If you are looking for a "bar" which isn't preceeded +"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 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)...|^.{0,2})bar/>. Sometimes it's still easier just to say: - if (/foo/ && $` =~ /bar$/) + if (/bar/ && $` !~ /foo$/) For lookbehind see below. @@ -653,9 +653,18 @@ first alternative includes everything from the last pattern delimiter the last alternative contains everything from the last "|" to the next pattern delimiter. For this reason, it's common practice to include alternatives in parentheses, to minimize confusion about where they -start and end. Note however that "|" is interpreted as a literal with -square brackets, so if you write C<[fee|fie|foe]> you're really only -matching C<[feio|]>. +start and end. + +Note that 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" +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, +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 |