Question

I’m implementing a regular expression parser by using algorithms in the dragon book. Now I can generate a DTran table and parse some basic regular expressions like: (a|b)*abb.

But I encountered a problem when implementing the . symbol, which matches any character. There is no text about it in the dragon book, is there some examples or articles about implementing it?

Answer 1

Solve it yourself, that is the fun of it. Below is not much an answer, but nevertheless.

1. Track the state/condition of a match for n number “.” so .[n] and then continue pattern matching.
2. Cheat and look ahead, not really a good idea. The since .[n] could be large, but likely not.

It is really rather similar to other pattern matching, like *, and remember multiple matches could occur on the same text; which is why maybe the left it out (A Dtran or state machine gets hard when the grammar is rich); plus it is really similar, but accepts an char. Heck you might have to switch to an NFA (nondeterministic finite automaton) to keep it simple.

Clearly I have not worked through the dragon book, but covered it in college, “The theory of computation”. I drew many state machines.