The current proposed named capture groups spec text specifies that we should throw a Syntax Error when processing the replacement string to String.prototype.replace() if there are malformed named capture groups references. This would be the first type of malformed replacement string text that would cause a throw. It seems out of place with the rest of the rules for various $<token> replacements. We have file an issue against the current spec text at https://github.com/tc39/proposal-regexp-named-groups/issues/29. If we don't prevail, then we'll need to add the throw(SyntaxError, ...) code.
<rdar://problem/34270676>
RegExp named capture groups proposal was changed in https://github.com/tc39/proposal-regexp-named-groups/commit/92ceba518c2ab0d2811c2efa8248ed1b3f8b5506: SyntaxErrors (nor any other errors) are no longer thrown. FIXME comments are removed in https://bugs.webkit.org/show_bug.cgi?id=205785.