java regular expression to discover regular expression

Question

I would like to build regular expression which checks if entered string is regular expression so if it contains |, [ and then ], ( and then ) for example:

A0[1-4].*Done or (L1 | L2).*Done'

I've build something simple like this: ".[\\[]..[\\]]" but it doesn't work at all even for simple examples. I tried with Java matcher and just simple String.matches()

What exactly should the regex match? A valid regex, or anything that somehow looks like a regex, i.e. contains typical regex-chars? — tobias_k
– tobias_k, Commented Jun 18, 2015 at 8:57
I doubt it's possible. The problem is regular expressions describe regular languages. And one of the properties of regular languages is that they are context-free, and regex, of course, is not context free. So, regex is not a regular language itself, so couldn't be described by regex. At the same time, Tagir's answer should suit you well. — mkrakhin
– mkrakhin, Commented Jun 18, 2015 at 8:58

Tagir Valeev · Accepted Answer · 2015-06-18 08:55:11Z

7

You can use Java own parser to check if the input is regular expression or not:

public boolean isRegExp(String pattern) {
    try {
        Pattern.compile(pattern);
        return true;
    }
    catch(PatternSyntaxException ex) {
        return false;
    }
}

answered Jun 18, 2015 at 8:55

Tagir Valeev

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