Check if two parts of a string are the same with regex (PHP)?

[Nostradomas]

using a regular expression in PHP with preg_replace_callback how can I check if two parts of the pattern are the same?\[code\]preg_replace_callback('/Blah: (.*?), BLah Blah (.+?), (**String Here Must Be Same as First Match**)/s', create_function(), $subject);\[/code\]Thanks,Matt