Regex: Lookahead and lookbehind, checking a . (dot) for use as decimal vs full-stop

Question

I've had a good look around the net for an answer to this, but can't seem to get it working.

I have developed the following regex:

    (?<![^\d][\\])[\.](?![\d])

The objective is to identify any '.' (dots) that have not been escaped or that are part of a decimal number.

ie)

• abc.co.uk, both dots should match
• ab0.co.uk, both dots should match
• abc.0.uk, both dots should match
• abc\.co.uk, only the second dot gets matched
• 0.00, dot should NOT match
• abc0.0.uk, first dot would NOT match (which is an acceptable outcome), second dot should

At moment it works for all the cases above, except:

• abc.0.uk, both dots should match

Any thoughts? It seems the look-behind is working correctly, however the look-ahead is not.

Am sure it'll be an easy one for any Regex gurus!

FYI. I'm developing this in .net 4

Answer 1

Try this one

(?<![\\\d])\.(?=\d)|(?<=[^\D\\])\.(?!\d)|(?<=[^\d\\])\.(?!\d)

See it here on Regexr

I broke it down in three steps.

1. Match if before is not a escape character and not a digit and behind is a digit.

2. Match if before is not a escape character and a digit and behind is not a digit

3. Match if before is not a escape character and not a digit and behind is not a digit