Sequential renaming "x+2" with Reg Exp - regex

Question

I have a batch of files with a pattern
AB 001 CD.txt
AB 002 FG.txt
AB 003 ID.txt ...
where the first 2 chars are constant, and the last 2 are all different.
I'd like to keep the first and last 2 chars intact, and just change the digits in the middle to (xxx + 2).
AB 001 CD.txt -> AB 003 DC.txt
AB 002 FG.txt -> AB 004 FG.txt
I am new to regular expressions, so the best I can do so far is find the digits with [0-9] but I need the replacement pattern.
Note that I'm only trying to test this with an application called "RegExr" to find a replacement match. Not sure this is doable with regex.

Answer 1

To match filenames you can use following regex:
^AB (\d{3}) .*$
However you cannot replace number part just with regex. You have to use some funcion specified for language you're using.
From now you have to get first matched group, convert it to an integer, increase with 2, add missing zeros to the left of value and replace with this string mentioned first group.

Answer 2

This isn’t really a good job for regular expressions – you cannot compute with regex. Numeric manipulation is therefore severely limited. Regex is the wrong technology for this job, I’m afraid. What you can do, though, is use a callback (depending on the language you use), something akin to the following (pseudocode, depends on the language you’re using):
result = regex_replace_callback('^(\w\w) (\d+) (\w\w).txt)$',
increment_match,
your_string)
function increment_match(match):
return match[1] + " " + (int(match[2]) + 2) + " " + match[3]