(?=[^,]) ,其中?=如何理解？

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Lookahead and lookbehind, collectively called “lookaround”, are zero-length assertions The difference is that lookaround actually matches characters, but then gives up the match, returning only the result: match or no match. They do not consume characters in the string, but only assert whether a match is possible or not. The regex q(?=u)i can never match anything. It tries to match u and i at the same position. If there is a u immediately after the q then the lookahead succeeds but then i fails to match u. If there is anything other than a u immediately after the q then the lookahead fails.

（？！） - negative lookahead

（？=） - positive lookahead （？=foo）bar —— 前面有foo的bar

（？<=） - positive lookbehind bar（？<=foo） —— 後面沒有foo的bar

（？>） - atomic group match first b match first b