new file mode 100644
--- /dev/null
+++ b/lib/mbscasestr.c
@@ -0,0 +1,145 @@
+/* Case-insensitive searching in a string.
+   Copyright (C) 2005-2007 Free Software Foundation, Inc.
+   Written by Bruno Haible <bruno@clisp.org>, 2005.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software Foundation,
+   Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
+
+#include <config.h>
+
+/* Specification.  */
+#include <string.h>
+
+#include <ctype.h>
+#include <stddef.h>  /* for NULL, in case a nonstandard string.h lacks it */
+
+#if HAVE_MBRTOWC
+# include "mbuiter.h"
+#endif
+
+#define TOLOWER(Ch) (isupper (Ch) ? tolower (Ch) : (Ch))
+
+/* Find the first occurrence of NEEDLE in HAYSTACK, using case-insensitive
+   comparison.
+   Note: This function may, in multibyte locales, return success even if
+   strlen (haystack) < strlen (needle) !  */
+char *
+strcasestr (const char *haystack, const char *needle)
+{
+  /* Be careful not to look at the entire extent of haystack or needle
+     until needed.  This is useful because of these two cases:
+       - haystack may be very long, and a match of needle found early,
+       - needle may be very long, and not even a short initial segment of
+         needle may be found in haystack.  */
+#if HAVE_MBRTOWC
+  if (MB_CUR_MAX > 1)
+    {
+      mbui_iterator_t iter_needle;
+
+      mbui_init (iter_needle, needle);
+      if (mbui_avail (iter_needle))
+	{
+	  mbchar_t b;
+	  mbui_iterator_t iter_haystack;
+
+	  mb_copy (&b, &mbui_cur (iter_needle));
+	  if (b.wc_valid)
+	    b.wc = towlower (b.wc);
+
+	  mbui_init (iter_haystack, haystack);
+	  for (;; mbui_advance (iter_haystack))
+	    {
+	      mbchar_t c;
+
+	      if (!mbui_avail (iter_haystack))
+		/* No match.  */
+		return NULL;
+
+	      mb_copy (&c, &mbui_cur (iter_haystack));
+	      if (c.wc_valid)
+		c.wc = towlower (c.wc);
+	      if (mb_equal (c, b))
+		/* The first character matches.  */
+		{
+		  mbui_iterator_t rhaystack;
+		  mbui_iterator_t rneedle;
+
+		  memcpy (&rhaystack, &iter_haystack, sizeof (mbui_iterator_t));
+		  mbui_advance (rhaystack);
+
+		  mbui_init (rneedle, needle);
+		  if (!mbui_avail (rneedle))
+		    abort ();
+		  mbui_advance (rneedle);
+
+		  for (;; mbui_advance (rhaystack), mbui_advance (rneedle))
+		    {
+		      if (!mbui_avail (rneedle))
+			/* Found a match.  */
+			return (char *) mbui_cur_ptr (iter_haystack);
+		      if (!mbui_avail (rhaystack))
+			/* No match.  */
+			return NULL;
+		      if (!mb_caseequal (mbui_cur (rhaystack),
+					 mbui_cur (rneedle)))
+			/* Nothing in this round.  */
+			break;
+		    }
+		}
+	    }
+	}
+      else
+	return (char *) haystack;
+    }
+  else
+#endif
+    {
+      if (*needle != '\0')
+	{
+	  /* Speed up the following searches of needle by caching its first
+	     character.  */
+	  unsigned char b = TOLOWER ((unsigned char) *needle);
+
+	  needle++;
+	  for (;; haystack++)
+	    {
+	      if (*haystack == '\0')
+		/* No match.  */
+		return NULL;
+	      if (TOLOWER ((unsigned char) *haystack) == b)
+		/* The first character matches.  */
+		{
+		  const char *rhaystack = haystack + 1;
+		  const char *rneedle = needle;
+
+		  for (;; rhaystack++, rneedle++)
+		    {
+		      if (*rneedle == '\0')
+			/* Found a match.  */
+			return (char *) haystack;
+		      if (*rhaystack == '\0')
+			/* No match.  */
+			return NULL;
+		      if (TOLOWER ((unsigned char) *rhaystack)
+			  != TOLOWER ((unsigned char) *rneedle))
+			/* Nothing in this round.  */
+			break;
+		    }
+		}
+	    }
+	}
+      else
+	return (char *) haystack;
+    }
+}