Matching numbers with regular expressions — only digits and commas

Question

I can\'t figure out how to construct a regex for the example values:

123,456,789
-12,34
1234
-8

Could you help me?

Answer 1

^[-+]?(\d{1,3})(,?(?1))*$

Debuggex Demo

So what does it?!

• ^ marks the beginning of the string
• [-+]? allows a minus or plus right after the beginning of the string
• (\d{1,3}) matches at least one and max three ({1,3}) digits (\d - commonly [0-9]) in a row and groups them (the parenthesises (...) builds the group) as the first group
• (,?(?1))* ok... let's break this down
  • (...) builds another group (not so important)
  • ,? matches a comma (if existent) right after the first sequence of digits
  • (?1) matches the pattern of the first group again (remember (\d{1,3})); in words: at this point the expression matches a sign (plus/minus/none) followed by a sequence of digits possibly followed by a comma, followed by another sequence of digits again.
  • (,?(?1))*, the * repeats the second part (comma & sequence) as often as possible
• $ finally matches the end of the string

the advantage of such expressions is, to avoid to define the same pattern within your expression again and again and again... well, a disadvantage is sometimes the complexity :-/