In general you need one alternative lookbehind for each possible length: (?:(?<=^)|(?<=\s)). In this case you can negate it to make the lookbehind always one character: (?<!\S).

In reply to Re: Match whitespace or start-of-string with lookbehind by ysth
in thread Match whitespace or start-of-string with lookbehind by BigLug

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