Question

I’m trying to solve this CodingBat problem:

Return true if the given string contains an appearance of “xyz” where the xyz is not directly preceeded by a period (.). So “xxyz” counts but “x.xyz” does not.

xyzThere(“abcxyz”) → true
xyzThere(“abc.xyz”) → false
xyzThere(“xyz.abc”) → true

I’m trying to solve this with a regex, but I’m unsure how to handle where the xyz is not directly preceeded by a period requirement.

My solution for the problem without the constraint is this:

public boolean xyzThere(String str) {
    return str.matches(".*xyz.*");
}

Any idea how to handle the said constraint with a regex?

Answer 1

A negated character class should do the trick: str.matches(".*(?:^|[^.])xyz.*")

Here we’re using a non capturing group (?:^|[^.]) to ensure that we match either at the start of the string ^, or at any position that isn’t a period [^.]