java regex to exclude specific strings from a larger one

Question

I have been banging my head against this for some time now: I want to capture all [a-z]+[0-9]? character sequences excluding strings such as sin|cos|tan etc. So having done my regex homework the following regex should work:

(?:(?!(sin|cos|tan)))\b[a-z]+[0-9]?

As you see I am using negative lookahead along with alternation - the \b after the non-capturing group closing parenthesis is critical to avoid matching the in of sin etc. The regex makes sense and as a matter of fact I have tried it with RegexBuddy and Java as the target implementation and get the wanted result but it doesn't work using Java Matcher and Pattern objects! Any thoughts?

cheers

Answer 1

The \b is in the wrong place. It would be looking for a word boundary that didn't have sin/cos/tan before it. But a boundary just after any of those would have a letter at the end, so it would have to be an end-of-word boundary, which is can't be if the next character is a-z.

Also, the negative lookahead would (if it worked) exclude strings like cost, which I'm not sure you want if you're just filtering out keywords.

I suggest:

\b(?!sin\b|cos\b|tan\b)[a-z]+[0-9]?\b

Or, more simply, you could just match \b[a-z]+[0-9]?\b and filter out the strings in the keyword list afterwards. You don't always have to do everything in regex.

Answer 2

So you want [a-z]+[0-9]? (a sequence of at least one letter, optionally followed by a digit), unless that letter sequence resembles one of sin cos tan?

\b(?!(sin|cos|tan)(?=\d|\b))[a-z]+\d?\b

results:

cos   - no match
cosy  - full match
cos1  - no match
cosy1 - full match
bla9  - full match
bla99 - no match