This fixes
    echo Y | LC_ALL=en_US.UTF-8 grep -i '[y]'
The expected output is:
    Y

Without this patch, it works on non UTF-8 environment, but fails on UTF-8
environment.

The definition of RE_ICASE comes from the glibc (/usr/include/regex.h)

Maybe lib/posix/regex.h should be removed to enforce the usage of the
glibc's regex.h

--- a/lib/posix/regex.h.orig	2004-01-05 12:09:12.984391131 +0000
+++ b/lib/posix/regex.h	2004-01-05 12:09:24.717990622 +0000
@@ -109,6 +109,10 @@
    treated as 'a\{1'.  */
 #define RE_INVALID_INTERVAL_ORD (RE_DEBUG << 1)

+/* If this bit is set, then ignore case when matching.
+   If not set, then case is significant.  */
+#define RE_ICASE (RE_INVALID_INTERVAL_ORD << 1)
+
 /* This global variable defines the particular regexp syntax to use (for
    some interfaces).  When a regexp is compiled, the syntax used is
    stored in the pattern buffer, so changing this does not affect
--- a/src/search.c	2009-02-11 17:15:24.000000000 +1100
+++ b/src/search.c	2009-02-11 17:23:48.000000000 +1100
@@ -172,10 +172,8 @@
   char const *motif = pattern;
 
   check_utf8 ();
-#if 0
   if (match_icase)
     syntax_bits |= RE_ICASE;
-#endif
   re_set_syntax (syntax_bits);
   dfasyntax (syntax_bits, match_icase, eolbyte);