Conditionals
The character '?' begins a conditional expression, the general form is:
?Ntrue-expression:false-expression
where N is decimal digit.
If sub-expression N was matched, then true-expression is evaluated and sent to output,
otherwise false-expression is evaluated and sent to output.
You will normally need to surround a conditional-expression with parenthesis in order to prevent ambiguities.
For example, the format string "(?1foo:bar)" will replace each match found with "foo" if the sub-expression $1 was matched,
and with "bar" otherwise.
For sub-expressions with an index greater than 9, or for access to named sub-expressions use:
?{INDEX}true-expression:false-expression
or
?{NAME}true-expression:false-expression


conditional expression ?Ntrue-expression:false-expression

In addition, conditional expressions of the following form are recognized:
?Ntrue-expression:false-expression
where N is a decimal digit representing a sub-match.
If the corresponding sub-match participated in the full match, then the substitution is true-expression.
Otherwise, it is false-expression. In this mode, you can use parens () for grouping.
If you want a literal paren, you must escape it as \(.