chnnvwshkld
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\[quote\] This question is an educational demonstration of the usage of lookahead, nested reference, and conditionals in a PCRE pattern to match ALL palindromes, including the ones that can't be matched by the recursive pattern given in the PCRE man page.\[/quote\]Examine this PCRE pattern in PHP snippet:\[code\]$palindrome = '/(?x)^ (?: (.) (?= .* ( \1 (?(2) \2 | ) ) $ ) )* .? \2?$/';\[/code\]This pattern seems to detect palindromes, as seen in this test cases (see also on ideone.com):\[code\]$tests = array( # palindromes '', 'a', 'aa', 'aaa', 'aba', 'aaaa', 'abba', 'aaaaa', 'abcba', 'ababa', # non-palindromes 'aab', 'abab', 'xyz',);foreach ($tests as $test) { echo sprintf("%s '%s'\n", preg_match($palindrome, $test), $test); }\[/code\]So how does this pattern work?NotesThis pattern uses a nested reference, which is a similar technique used in How does this Java regex detect palindromes?, but unlike that Java pattern, there's no lookbehind (but it does use a conditional).Also, note that the PCRE man page presents a recursive pattern to match some palindromes:\[code\]# the recursive pattern to detect some palindromes from PCRE man page^(?
(.)(?1)\2|)|((.)(?3)\4|.))$\[/code\]The man page warns that this recursive pattern can NOT detect all palindromes (see: Why will this recursive regex only match when a character repeats 2n - 1 times?http://stackoverflow.com/questions/...nly-match-when-a-character-repeats-2n-1-times and also on ideone.com), but the nested reference/positive lookahead pattern presented in this question can.
(.)(?1)\2|)|((.)(?3)\4|.))$\[/code\]The man page warns that this recursive pattern can NOT detect all palindromes (see: Why will this recursive regex only match when a character repeats 2n - 1 times?http://stackoverflow.com/questions/...nly-match-when-a-character-repeats-2n-1-times and also on ideone.com), but the nested reference/positive lookahead pattern presented in this question can.