Why doesn't this regex work as expected in Java?

Matcher.matches() checks to see if the entire input string is matched by the regex.

Since your regex only matches the very first character, it returns false.

You'll want to use Matcher.find() instead.

Granted, it can be a bit tricky to find the concrete specification, but it's there:

• String.matches() is defined as doing the same thing as Pattern.matches(regex, str).
• Pattern.matches() in turn is defined as Pattern.compile(regex).matcher(input).matches().
  • Pattern.compile() returns a Pattern.
  • Pattern.matcher() returns a Matcher
• Matcher.matches() is documented like this (emphasis mine):

  Attempts to match the entire region against the pattern.

The matches method matches your regex against the entire string.

So try adding a .* to match rest of the string.

"#This is a comment in a file".matches("^#.*")

which returns true. One can even drop all anchors(both start and end) from the regex and the match method will add it for us. So in the above case we could have also used "#.*" as the regex.