User Guide : Using Content Extraction Language : Language Syntax and Examples : ?
 
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?
The question mark ( ? ) is a metacharacter that is used in regular expressions. It matches zero or one of the characters that it follows. It can follow a regular expression that evaluates to a character.
Example
/mar?y/
will match may or mary but does not match marry
/^[-+]?[0-9]$/
will match an optional signed number.
For information on how they participated in this regular expression, see ^ [ ] - and $.