advanced regex.txt

lookahead positive
/foo(?=bar)/   match foo in Hello foobar

lookahead negative
/foo(?!bar)/   matches 'foo', only if it is NOT followed by 'bar'

Lookbehind positive
/(?<=foo)bar/  matches 'bar' only if it is preceded by 'foo'

Lookbehind negative
/(?<!foo)bar/  matches 'bar' if it is NOT preceded by 'foo'

Collectively, lookbehinds and lookaheads are known as lookarounds.

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Regular expressions provide the functionality for checking certain conditions.

(?(condition)true-pattern | false-pattern)
or
(?(condition)true-pattern)

The condition can be a number. In which case it refers to a previously captured subpattern.

$pattern = '/^(<)?[a-z]+(?(1)>)$/';
 
preg_match($pattern, '<test>'); // true
preg_match($pattern, '<foo'); // false
preg_match($pattern, 'bar>'); // false
preg_match($pattern, 'hello'); // true

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http://www.rexegg.com/regex-disambiguation.html