monger:

You said you don't want the first set of date/time info, nor the "WARNING" ... so why capture them at all? Use non_captureing parens,     (:...) for that which you are using merely as a marker for the string you want.

Please read the two items surrounding kaif's question... (Aargh; seem to have sent reply to kaif's to bitbucket, so repeating below) and more particularly (or, at least, even more diligently) the suggestions from holli, et al.

Update:

 (?=WARNING) is a lookahead (aka lookaround), which

• matches at the POSITION or "location" before "WARNING" (ie, uses "WARNING" as a marker to delimit the prior part(s) of the regex; you can think of this as a way to limit greediness, tho that's very sloppy language) but...
• does not capture "WARNING"!

so...

the second "WARNING" in the regex matches the word itself which therefore gets included (along with the trailing \s, space) in the NON_capture,  (?:...)

HTH