php - Compilation failed: unmatched parentheses at offset -

i writing regular expression, when check following regex online, works correctly. however, return error in php. why that?

preg_match_all('/^[([a-z]+\[[a-z| |,]+\])\,?]+$/', $methods, $result);

return

severity: warning  message: preg_match_all() [function.preg-match-all]: compilation failed: unmatched parentheses @ offset 23

i tried 1 too:

preg_match_all('/^[([a-z]+\\[[a-z| |,]+\\])\\,?]+$/', $methods, $result);

if change outer character class ([]) character group (parentheses) compile, im not sure kind of string trying match:

/^(([a-z]+\[[a-z| |,]+\])\,?)+$/

like in example

i think caused because cant put nested character class because lose meaning regex ends in 3 parts this:

[([a-z]+  \[[a-z| |,]+  \])\,?]+

ending in unmatched enter parentheses

KHS