I'm trying to craft two regular expressions that will match URIs. These URIs are of the format: /foo/someVariableData
and /foo/someVariableData/bar/someOtherVariableData
I need two regexes. Each needs to match one but not the other.
The regexes I originally came up with are:
/foo/.+
and /foo/.+/bar/.+
respectively.
I think the second regex is fine. It will only match the second string. The first regex, however, matches both. So, I started playing around (for the first time) with negative lookahead. I designed the regex /foo/.+(?!bar)
and set up the following code to test it
public static void main(String[] args) {
String shouldWork = "/foo/abc123doremi";
String shouldntWork = "/foo/abc123doremi/bar/def456fasola";
String regex = "/foo/.+(?!bar)";
System.out.println("ShouldWork: " + shouldWork.matches(regex));
System.out.println("ShouldntWork: " + shouldntWork.matches(regex));
}
And, of course, both of them resolve to true
.
Anybody know what I'm doing wrong? I don't need to use Negative lookahead necessarily, I just need to solve the problem, and I think that negative lookahead might be one way to do it.
Thanks,
Try
String regex = "/foo/(?!.*bar).+";
or possibly
String regex = "/foo/(?!.*\\bbar\\b).+";
to avoid failures on paths like /foo/baz/crowbars
which I assume you do want that regex to match.
Explanation: (without the double backslashes required by Java strings)
/foo/ # Match "/foo/"
(?! # Assert that it's impossible to match the following regex here:
.* # any number of characters
\b # followed by a word boundary
bar # followed by "bar"
\b # followed by a word boundary.
) # End of lookahead assertion
.+ # Match one or more characters
\b
, the "word boundary anchor", matches the empty space between an alphanumeric character and a non-alphanumeric character (or between the start/end of the string and an alnum character). Therefore, it matches before the b
or after the r
in "bar"
, but it fails to match between w
and b
in "crowbar"
.
Protip: Take a look at http://www.regular-expressions.info - a great regex tutorial.