How can I express such requirement using Java regular expression?

Question

I need to check that a file contains some amounts that match a specific format:

• between 1 and 15 characters (numbers or ",")
• may contains at most one "," separator for decimals
• must at least have one number before the separator
• this amount is supposed to be in the middle of a string, bounded by alphabetical characters (but we have to exclude the malformed files).

I currently have this:

\d{1,15}(,\d{1,14})?

But it does not match with the requirement as I might catch up to 30 characters here.
Unfortunately, for some reasons that are too long to explain here, I cannot simply pick a substring or use any other java call. The match has to be in a single, java-compatible, regular expression.

Answer 1

^(?=.{1,15}$)\d+(,\d+)?$

• ^ start of the string
• (?=.{1,15}$) positive lookahead to make sure that the total length of string is between 1 and 15
• \d+ one or more digit(s)
• (,\d+)? optionally followed by a comma and more digits
• $ end of the string (not really required as we already checked for it in the lookahead).

You might have to escape backslashes for Java: ^(?=.{1,15}$)\\d+(,\\d+)?$

update: If you're looking for this in the middle of another string, use word boundaries \b instead of string boundaries (^ and $).

\b(?=[\d,]{1,15}\b)\d+(,\d+)?\b

For java:

"\\b(?=[\\d,]{1,15}\\b)\\d+(,\\d+)?\\b"

More readable version:

"\\b(?=[0-9,]{1,15}\\b)[0-9]+(,[0-9]+)?\\b"