Regex is done in custom hardware, so it's not just a standard off-the-shelf engine, but for these purposes it's close enough to PCRE. In fact since you're just doing a string match simply using Automated_Clearing_House_transaction will work.
I created and tested a signature with the following config:
and it worked on a test url:
The XML for the signature is:
drogers@Captain.America# show threats vulnerability 42001 signature
<entry name="And Condition 1">
<entry name="Or Condition 1">
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