A Regular Expression Matcher (2007)

I have a re.c file in my best-of collection of code examples locally - but I can't seem to find the original source quickly - nevertheless it was written by Russ Cox[0] and he's spent a lot of time making regular expressions accessible and elegant, his articles are a great resource. [0] https://swtch.com/~rsc/regexp/

Can someone explain to me why the matchstar function takes its first param as an integer? to me it should be a char. int matchstar(int c, char *regexp, char *text) { do { /* a * matches zero or more instances */ if (matchhere(regexp, text)) return 1; } while (*text != '\0' && (*text++ == c || c == '.')); return 0; }

It's very nice pattern-matching code, but it isn't a regular expression matcher. The class of patterns it's able to match is much smaller than the class of all regular expressions -- not because of missing "sugar" (character classes, +, non-greedy wildcards, ...) but because it doesn't support groups or the | operator. As a result, the language supported by this matcher has, e.g., no way to match the pattern (ab)* or the pattern a|b. It's much, much less powerful than…

"[star] matches zero or more occurrences of the previous character" …those aren't regular expressions. Those are fancy globs. Regular expressions have a formal definition that is strictly more expressive than that. They mention they use recursion (which you can use to actually implement regular expressions, albeit poorly) but… …they use the recursion as a way of implementing a state machine in C? You can get away with this (and it's somewhat common) in a language like Lisp or ML that…