How to check if the string is a regular expression or not

Question

I have a string. How I can check if the string is a regular expression or contains regular expression or it is a normal string?

Answer 1

The only reliable check you could do is if the String is a syntactically correct regular expression:

boolean isRegex;
try {
  Pattern.compile(input);
  isRegex = true;
} catch (PatternSyntaxException e) {
  isRegex = false;
}

Note, however, that this will result in true even for strings like Hello World and I'm not a regex, because technically they are valid regular expressions.

The only cases where this will return false are strings that are not valid regular expressions, such as [unclosed character class or (unclosed group or +.

Answer 2

This is ugly but will detect simple regular expressions (with the caveat they must be designed for Java i.e. have the relevant back-slash character escaping).

public boolean isRegex(final String str) {
    try {
        java.util.regex.Pattern.compile(str);
        return true;
    } catch (java.util.regex.PatternSyntaxException e) {
        return false;
    }
}

Answer 3

Maybe you'd try to compile that regular expression using regexp package from Apache ( http://jakarta.apache.org/regexp/ ) and, if you get an exception then that's not a valid regexp so you'd say it's a normal string.

boolean validRE = true;
try {
    RE re = new RE(stringToCheck);
} catch (RESyntaxException e) {
    validRE = false;
}

Obviously, the user would have typed an invalid regexp and you'd be handling it as a normal string.

Answer 4

there is no difference between a 'normal' sting and a regular expression. A regular expression is just a normal string which is used as a pattern to match occurrences of the pattern in another string.

As others have pointed out, it is possible that the string might not be a valid regular expression, but I think that is the only check you can do. If it is valid then there is no way to know if it is a regular expression or just a normal string because it will be a regular expression

It is just a normal string which is interpreted in a specific way by the regex engine.

for example "blah" is a regular expression which will only match the string "blah" where ever it occurs in another string.

When looked at this way, you can see that a regular expression does not need to contain any of the 'special characters' that do more advanced pattern matching, and it will only match the string in the pattern