You're welcome, Garry. Stefan - a simple check for whether pattern has any regex metacharacters (e.g. '.', '*', etc.) can decide if a pattern is definitely not a regular expression. If there are any metacharacters in pattern, well only the user can know and there would have to be a command-line switch to force one interpretation vs. another.
In terms of absolute maximum performance, one usually wants to not do the search "line at a time" at all. Rather, one wants to use an assembly-optimized "strchr"/"memchr"-like search on the biggest chunks of input possible. In this context that means scanning the input for the least likely character that is definitely necessary for a match. There may be characters much less likely than newlines, and so a big boost may be possible (though note that input data sets do vary). Once that byte offset is known, from there one determines if it is part of a real match. That is all quite a bit more bookkeeping/logic/hassle, obviously.
