Recently I read the source code of Clang, and found that it just do the lex thing by reading characters one by one and manually do the matching. But both my teacher when I'm in college and the book "Compilers Principles, Techniques, and Tools" take lots time to illustrate how to build a regular expression state machine, and I remember the tool Flex also build a state machine (correct me if I'm wrong). I think it's easy to lost in the manually matching code and if there's something changed in a language, it is a little difficult to update the matching code.
So why nowadays people prefer to manually do the matching rather than use regular expression? Is this because the table generated by the regex tool too big if the token table is complicated? Or is this an efficiency trade-off?
p.s. Evidence for "more and more":
Gcc (as long as they abandoned flex/bison)
What interesting is, the author of protobuf wants to use regular expression, but give up due to open source project get better keep independent, I think this counts a reason.