Change "id" to "idx" to be Objective C++ compatible.

Fixes #2306.
Fixes #4132.
Fixes #4191.

[SVN r61789]

libs/regex

@@ -1 +1 @@
-Subproject commit 87d5fd1421cf6c0c2e0acb11189736befdf5b0b7
+Subproject commit fa96f4edf1d1e7362c822499bb9ee430e5585dc7