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Regex

3.4.3 The Match-zero-or-one Operator (? or \?)

If the syntax bit RE_LIMITED_OPS is set, then Regex doesn't recognize this operator. Otherwise, if the syntax bit RE_BK_PLUS_QM isn't set, then `?' represents this operator; if it is, then `\?' does.

This operator is similar to the match-zero-or-more operator except that it repeats the preceding regular expression once or not at all; see section 3.4.1 The Match-zero-or-more Operator (*), to see what it operates on, how some syntax bits affect it, and how Regex backtracks to match it.

For example, supposing that `?' represents the match-zero-or-one operator; then `ca?r' matches both `car' and `cr', but nothing else.