Unfortunately, this is virtually not fixable. With a JS regexp, there is no way to get the group count without having a successful match. So either we parse the regexp on an ad hoc basis to get that count (hard), or we derive automatically a matching string from the regexp to match against it (which requires to parse the regex in the first place).

Unless someone else has an idea...

Yet another issue that would be solved by our own regex parser as proposed in #1201

I played around with JS RegExps and read some ref doc - definitely doesn't seem possible.

Regex continues to be the largest source of incompatible Scala/Scala.JS behaviour. It's a significant problem because you only hit the errors at runtime - compilation & linking passes silently. The only way to catch incompatibilities like this one, unsupported regex like (?<!x), etc is probably going to along the lines of a custom regex lib like @gzm0 suggested. I don't know what else can be done. It might not be pleasant but it is better than a partially-discovered mountain of runtime-only errors, which is what we have today.

I'll close this as Won't Fix. If it must be fixed, it will go into the larger scheme of things of fixing #1201 anyway.

For people having a problem with groupCount, my fix is the following:

def countGroups(m: Regex.Match): Int = {
   try {m.groupCount} catch { case e: Throwable =>
     Stream.from(0).takeWhile(i => try { val x = m.group(i); x != null } catch { case e: Throwable => false} ).length - 1
   }
 }

If the call to groupCount fails, it tries to enumerate the groups and return the inferred count. It saved my tool (scalajs 0.6.19)

Thanks @MikaelMayer for the fix, it works for me too.

@sjrd is this something we could fold into the stldlib .groupCount?

I don't think so, because the workaround builds on top of Scala's Regex.Match, not on top of Java's regex.Matcher.

This was in fact fixed in e5d3d6b as part of fixing #3280, which should have been marked as a duplicate of this one, but we did not notice.