I'm trying to create a regexp_instr formula to identify a value in a multi-select field but am coming up short when it comes to the lookahead and lookbehind assertions. are these possible in NetSuite? Essentially, I'm looking for if an exact value exists anywhere in the string. INSTR would lead to false positives, so I need to account for single values (first expr), if the value is the start of the string (2nd expr), if the value is the end of the string (3rd expr), or if the value is nested in the middle somewhere (4th expr). I've tried escaping and not escaping the commas (snip below is escaped). I'm newish to regex but I go this working on a regex testing site, but of course, moving to NS posed problems and I can't find any complex examples of this online. Appreciate the help!

REGEXP_INSTR({custrecord_acs_certificate_item}, '^115479$|^115479(?=\,)|(?<=\,)115479$|(?<=\,)115479(?=\,)')