Why doesn't positive lookahead work as first capture group?
javascript, regex, regex-lookarounds
Solution
Change your regex like below and then grab the strings you want from group index 1 and 2.
(?:_missing_:|_exists_:)([a-z1-9]+)|([a-z1-9]+)(?=:)
You don't need to include the non-capturing group `(?:_missing_:|_exists_:)` inside a capturing group. This is the reason for returning `missing:title` instead of `title` . And also Capturing group for `[a-z1-9]+` would be enough.
DEMO
Problem
I'm using the following regex; ``` ((?:_missing_:|_exists_:)[a-z0-9]+)|(([a-z0-9]+)(?=:)) ``` To match a lucene query string; ``` _missing_:title age:(>=10 AND < 20) AND age:123 AND _exists_:title123 ``` The first non-capture group is not respected and returns `_missing_:title` and not `title`. Using a positive lookahead makes the entire regex fail to match anything. It should return the following array; ``` ['title', 'age', 'age', 'title123'] ```