Question

I’m trying to pull out all strings which have an even number of B’s and an odd number of C’s. I have the regexes to match odd A’s and even B’s but I cannot get the two to work together. The strings are delimited by whitespace (tabs, newlines, spaces).

e.g.

XABBAC     ABCDEBCC ABSDERERES ABBAAJSER     HGABAA

I have for odd A’s

\b[^A]*A([^A]*A[^A]*A)*[^A]*\b

And for even B’s

\b[^B]*(B[^B]*B[^B]*)*[^B]*\b

I know I need to use +ve lookahead and have tried:

\b(?=[^A]*A([^A]*A[^A]*A)*[^A]*\b)[^B]*(B[^B]*B[^B]*)*[^B]*\b

but it doesn’t work – does anybody know why?

Answer 1

The problem is that your regexes (regexen?) can match zero characters – \b\b will match on a single word boundary, and so will \b{someregexthatcanmatchzerocharacters}\b.