here's some stuff from the makers of Regex Buddy. These came from the library so i'm confident they have been thoroughly tested.
Number: Currency amount (cents mandatory)
Optional thousands separators; mandatory two-digit fraction
Match; JGsoft:
^[+-]?[0-9]{1,3}(?:,?[0-9]{3})*.[0-9]{2}$
Number: Currency amount (cents optional)
Optional thousands separators; optional two-digit fraction
Match; JGsoft:
^[+-]?[0-9]{1,3}(?:,?[0-9]{3})*(?:.[0-9]{2})?$
Number: Currency amount US & EU (cents optional)
Can use US-style 123,456.78 notation and European-style 123.456,78 notation. Optional thousands separators; optional two-digit fraction
Match; JGsoft:
^[+-]?[0-9]{1,3}(?:[0-9]*(?:[.,][0-9]{2})?|(?:,[0-9]{3})*(?:.[0-9]{2})?|(?:.[0-9]{3})*(?:,[0-9]{2})?)$
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