Grabbed from Debian:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=249245

To fix Gentoo bug:
http://bugs.gentoo.org/show_bug.cgi?id=76192

--- grep-2.5.1.orig/src/dfa.c	2004-10-19 02:07:23.000000000 +0900
+++ grep-2.5.1/src/dfa.c	2004-10-19 02:24:28.000000000 +0900
@@ -652,6 +652,20 @@
 	  REALLOC_IF_NECESSARY(work_mbc->chars, wchar_t, chars_al,
 			       work_mbc->nchars + 1);
 	  work_mbc->chars[work_mbc->nchars++] = (wchar_t)wc;
+	  if (case_fold && (iswlower((wint_t) wc) || iswupper((wint_t) wc)))
+	    {
+		wint_t altcase;
+
+		altcase = wc;		/* keeps compiler happy */
+		if (iswlower((wint_t) wc))
+		  altcase = towupper((wint_t) wc);
+		else if (iswupper((wint_t) wc))
+		  altcase = towlower((wint_t) wc);
+
+		REALLOC_IF_NECESSARY(work_mbc->chars, wchar_t, chars_al,
+			       work_mbc->nchars + 1);
+		work_mbc->chars[work_mbc->nchars++] = (wchar_t) altcase;
+	    }
 	}
     }
   while ((wc = wc1) != L']');