Match-zero-or-one Operator

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The Match-zero-or-one Operator (`?' or `\?')
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  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 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.