Brilliant, thank you all for your speedy replies. FWIW the regex was simply hanging apache with real data at my end (some lists have 300K points) but the chromatic and ikegami solutions are definitely helping.

It has highlighted that actually the real problem is algorithmic rather than performance of this section of code as the speed up, whilst beneficial, isn't the main problem. I can think of a way of whittling down the data to test in another part of the code so I'll try that out next. Again, many thanks - and ikegami's code will remain in as the winner so far. Roger