I think I must be missing something. I was using a regex to replace a character in a text field. I needed to replace a comma with a semi-colon and a space. It added the semi-colon but not the space.
Not sure what I am missing here.
It seems you cannot add a space with a regular expression action as you have found out.
The only other way I got this to work was to use the Regular Expression action to split the string based on the comma. This outputs into a collection variable. I then used two Get Item From collection actions to get the two parts on either side of the comma.
Then used a build string to put them back togeter with a semicolon and a space.
Workflow looked like this:
Workflow also attached that splits my name Simon,Muntz and changes it to Simon; Muntz and logs it to the history.