Question

What I need is to check whether a given string partially matches a given regex. For example, for the regex ab[0-9]c, the strings “a”, “ab”, “ab3”, and “b3c” would “match”, but not the strings “d”, “abc”, or “a3c”. What I’ve been doing is the clunky a(?:b(?:[0-9](?:c)?)?)? (which only works for some of the partial matches, specifically those which “begin” to match), but since this is part of an API, I’d rather give the users a more intuitive way of entering their matching regexps.

In case the description’s not very clear (and I realize it might not be!), this will be used for validating text input on text boxes. I want to prevent any editing that would result in an invalid string, but I can’t just match the string against a regular regex, since until it’s fully entered, it would not match. For example, using the regex above (ab[0-9]c), when I attempt to enter ‘a’, it’s disallowed, since the string “a” does not match the regex.

Basically, it’s a sort of reverse startsWith() which works on regexps. (new Pattern("ab[0-9]c").startsWith("ab3") should return true.)

Any ideas?

Answer 1

Is Matcher.hitEnd() what you’re looking for?

Pattern thePattern = Pattern.compile(theRegexString);
Matcher m = thePattern.matcher(theStringToTest);
if (m.matches()) {
    return true;
}
return m.hitEnd();