"Christos Georgiou" <[EMAIL PROTECTED]> wrote in message news:[EMAIL PROTECTED] > On Thu, 26 Jan 2006 14:09:54 GMT, rumours say that "Roger L. Cauvin" > <[EMAIL PROTECTED]> might have written: > >>Say I have some string that begins with an arbitrary sequence of >>characters >>and then alternates repeating the letters 'a' and 'b' any number of times, >>e.g. >> >>"xyz123aaabbaabbbbababbbbaaabb" >> >>I'm looking for a regular expression that matches the first, and only the >>first, sequence of the letter 'a', and only if the length of the sequence >>is >>exactly 3. >> >>Does such a regular expression exist? If so, any ideas as to what it >>could >>be? > > Is this what you mean? > > ^[^a]*(a{3})(?:[^a].*)?$
Close, but the pattern should allow "arbitrary sequence of characters" that precede the alternating a's and b's to contain the letter 'a'. In other words, the pattern should accept: "xayz123aaabbab" since the 'a' between the 'x' and 'y' is not directly followed by a 'b'. Your proposed pattern rejects this string. -- Roger L. Cauvin [EMAIL PROTECTED] (omit the "nospam_" part) Cauvin, Inc. Product Management / Market Research http://www.cauvin-inc.com -- http://mail.python.org/mailman/listinfo/python-list
