Is it possible to have regexp that matches all valid regular expressions?

Is it possible to detect if a given string is valid regular expression, using just regular expressions?

Say I have some strings, that may or may not be a valid regular expressions. I'd like to have a regular expres开发者_开发问答sion matches those string that correspond to valid regular expression. Is that possible? Or do I have use some higher level grammar (i.e. context free language) to detect this? Does it affect if I am using some extended version of regexps like Perl regexps?

If that is possible, what the regexp matching regexp is?

No, it is not possible. This is because valid regular expressions involve grouping, which requires balanced parentheses.

Balanced delimiters cannot be matched by a regular expression; they must instead be matched with a context-free grammar. (The first example on that article deals with balanced parentheses.)

See an excellent write-up here:

Regular expression for regular expressions?

The answer is that regexes are NOT written using a regular grammar, but a context-free one.

If your question had been "match all valid regular expressions", the answer is (perhaps surprisingly) 'yes'. The regular expression .* match all valid (and non-valid) regular expressions, but is pretty useless for determining if you're looking at a valid one.

However, as the question is "match all and only valid regular expressions", the answer is (as DVK and Platinum Azure" have said 'no'.