If the group doesn't participate in a match, i.e., it is part of an
alternative not taken or a repetition operator allows zero
repetitions of it, then the back reference makes the whole match
fail. For example, `(one()|two())-and-(three\2|four\3)' matches
`one-and-three' and `two-and-four', but not `one-and-four' or
`two-and-three'. For example, if the pattern matches `one-and-',
then its group 2 matches the empty string and its group 3 doesn't
participate in the match. So, if it then matches `four', then
when it tries to back reference group 3--which it will attempt to
do because `\3' follows the `four'--the match will fail because
group 3 didn't participate in the match.
