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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 expression 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?
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No, if you are strictly speaking about regular expressions and not including some regular expression implementations that are actually context free grammars. There is one limitation of regular expressions which makes it impossible to write a regex that matches all and only regexes.
The method str. match(regexp) finds matches for regexp in the string str . If the regexp has flag g , then it returns an array of all matches as strings, without capturing groups and other details. If there are no matches, no matter if there's flag g or not, null is returned.
Regex isn't suited to parse HTML because HTML isn't a regular language. Regex probably won't be the tool to reach for when parsing source code. There are better tools to create tokenized outputs. I would avoid parsing a URL's path and query parameters with regex.
An empty regular expression matches everything.
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.)
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