Regular expression match only if subpattern doesn't match

Question

Hello

I'm trying to match C style comments form a file, but only if the comment don't start with a certain labels introduced by @

For example from

/* some comment to match */
/* another comment.
this should match also */
/*@special shouldn't match*/

Is this possible using regular expressions only?

I'm trying this using JavaScript implementation of regular expressions.

Answer 1

You could start with something like this:

/\*[^@]

But in general, you don't watch to match C-style comments with regular expressions, because of nasty corner-cases. Consider:

"foo\" /* " " */ "

There's no comment in that code (it's a compile-time concatenation of two string literals), but you're not going to have much luck parsing it without a real parser. (Technically, you could use a regular expression, because you only need a simple finite state machine. But it's a very disgusting regular expression.)

Answer 2

/\*\s*(?!@)(?:(?!\*/).)*\*/

Breaks down as:

/\*               // "/*"
\s*               // optional space
(?!@)             // not followed by "@"
(?:               // don't capture...
   (?!\*/).       // ...anything that is not "*/"
)*                // but match it as often as possible
\*/               // "*/"

Use in "global" and "dotall" mode (e.g. the dot should match new lines as well)

The usual word of warning: As with all parsing jobs that are executed with regular expressions, this will fail on nested patterns and broken input.

emk points out a nice example of (otherwise valid) input that will cause this expression to break. This can't be helped, regex is not for parsing. If you are positive that things like this can never occur in your input, a regex might still work for you.

Answer 3

use negative lookahead