What is the lib/algorithm for matching multiple regular expression against a single target string in C?

Question

I want to match multiple regular expressions against a single string & stop when the first regular expression matches.

I am exploring few solutions from here http://sljit.sourceforge.net/regex_perf.html

but none of them seem to take into consideration match of multiple regular expression against single string.

Is there any solution to speed this up?

Answer 1

You could just use alternation. That is, if you're looking for the expressions \a+b\ and \[a-z0-9]+xyz\, you could write a single regular expression with grouping: \(a+b)|([a-z0-9]+xyz)\. The regex engine will return the first match it finds.

The Unix fgrep tool does what you're looking for. If you give it a list of expressions to find, it will find all occurrences in a single scan of the file. Dr. Dobb's Journal published an article about it, with C source, sometime back in the late '80s. A quick search reveals that the article was called Parallel pattern matching and fgrep, by Ian Ashdown. I didn't find the source, but I didn't look all that hard. Given a little time, you might have more luck.