Question

I have developed a UI that allows users to define a regex pattern and then a replacement string that is used by appendReplacement().

e.g.

• Pattern – 7(.*)
• Replace pattern – $1

So 71234 would yield 1234.

Is there anyway I can verify that the input pattern and replace pattern are compatible without the user having to enter a matching string? For example if the replace pattern was $1$2 then this would throw an exception at runtime. Can I check for this in advance?

EDIT:

Final solution was to do as aioobe suggested and get the group count by creating a dummy Matcher (not sure why this method isn’t on Pattern TBH) and then manually parsing the replace string manually to find which $ group references are present. Bit fiddly but works.

Answer 1

Well, you could do something like:

yourPattern.matcher("").groupCount();

groupCount() will return the number of capturing groups (which at least can be thought of as an upper limit of what the user may refer to).