用于多行注释的 Unix Flex 正则表达式

Question

I am making a Lexical Analyzer using Flex on Unix. If you've ever used it before you know that you mainly just define the regex for the tokens of whatever language you are writing the Lexical Analyzer for. I am stuck on the final part. I need the correct Regex for multi-line comments that allows something like

/* This is a comment \*/

but also allows

/* This **** //// is another type of comment */

Can anyone help with this?

Answer 1

You don't match C style comments with a simple regular expression in Flex; they require a more complex matching method based on start states. The Flex FAQ says how (well, they do for the /*...*/ form; handling the other form in just the <INITIAL> state should be simple).

Answer 2

If you're required to make do with just regex, however, there is indeed a not-too-complex solution:

---

"/*"( [^*] | (\\*+[^*/]) )*\\*+\\/

---

The full explanation and derivation of that regex is excellently elaborated upon here .
In short:

"/*" marks the start of the comment

( [^*] | (\\*+[^*/]) )* says accept all characters that are not * (the [^*] ) or accept a sequence of one or more * as long as the sequence does not have a '*' or a /' following it (the (*+[^*/])). This means that all ******... sequences will be accepted except for *****/ since you can't find a sequence of * there that isn't followed by a * or a /.

The *******/ case is then handled by the last bit of the RegEx which matches any number of * followed by a / to mark the end of the comment ie \\*+\\/

Answer 3

http://www.lysator.liu.se/c/ANSI-C-grammar-l.html does:

"/*"            { comment(); }

comment() {
    char c, c1;

loop:
    while ((c = input()) != '*' && c != 0)
        putchar(c);

    if ((c1 = input()) != '/' && c != 0) {
        unput(c1);
        goto loop;
    }

    if (c != 0)
        putchar(c1);
}

A question which would also solve this is How do I write a non-greedy match in LEX / FLEX?

Answer 4

i don't know flex but i do know regexs. /\\/\\*.*?\\*\\//s should match both types (in PCRE), but if you need to differentiate them in your analyser, you may want to then iterate the list of matches to see if they're the second type with /\\*\\*\\s+\\/{4}/