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Pretty much an editorial omission in FO30 and FO31. The description of the FORX0001 error reads: Raised by regular expression functions such as fn:matches and fn:replace if the regular expression flags contain a character other than imsx But the WG introduced the "q" flag as a valid flag for quoted expressions, so this is now "imsxq". Also note that the "." is missing from the end of the sentence ;). The bug is present in both FO30 and FO31.
Thanks, fixed.