Question

How do I replace the whitespace beginning from the last comma in the following string:

foo, bar, 2 3 4 5

should yield:

foo ,bar ,2345

The whitespace up to the second comma should remain intact.

Answer 1

Search for

\s+(?=[^,]*$)

and replace with nothing. This matches whitespace characters only if there are no commas further ahead in the string.

If you want to implement the rule “all whitespace after the second comma in the string, regardless of whether there are more commas after that”, then you need a regex engine that can handle indefinite repetition inside lookbehind assertions; currently only .NET and JGSoft engines do this. If you’re using one of those, fine:

(?<=(?:,[^,]*){2})\s+

matches whitespace if it’s preceded by at least two commas.