Index: src/OFMutableString.m
==================================================================
--- src/OFMutableString.m
+++ src/OFMutableString.m
@@ -243,8 +243,61 @@
 		@throw [OFInvalidEncodingException newWithClass: isa];
 
 	while (--p >= string)
 		*p = tolower((int)*p);
 
+	return self;
+}
+
+- replaceOccurrencesOfString: (OFString*)str
+		  withString: (OFString*)repl
+{
+	const char *str_c = [str cString];
+	const char *repl_c = [repl cString];
+	size_t str_len = [str length];
+	size_t repl_len = [repl length];
+	size_t i, last, tmp_len;
+	char *tmp;
+
+	if (str_len > length)
+		return self;
+
+	tmp = NULL;
+	tmp_len = 0;
+
+	for (i = 0, last = 0; i <= length - str_len; i++) {
+		if (memcmp(string + i, str_c, str_len))
+			continue;
+
+		@try {
+			tmp = [self resizeMem: tmp
+				       toSize: tmp_len + i - last +
+					       repl_len + 1];
+		} @catch (OFException *e) {
+			[self freeMem: tmp];
+			@throw e;
+		}
+		memcpy(tmp + tmp_len, string + last, i - last);
+		memcpy(tmp + tmp_len + i - last, repl_c, repl_len);
+		tmp_len += i - last + repl_len;
+		i += str_len - 1;
+		last = i + 1;
+	}
+
+	@try {
+		tmp = [self resizeMem: tmp
+			       toSize: tmp_len + length - last + 1];
+	} @catch (OFException *e) {
+		[self freeMem: tmp];
+		@throw e;
+	}
+	memcpy(tmp + tmp_len, string + last, length - last);
+	tmp_len += length - last;
+	tmp[tmp_len] = 0;
+
+	[self freeMem: string];
+	string = tmp;
+	length = tmp_len;
+
 	return self;
 }
 @end

Index: src/OFString.h
==================================================================
--- src/OFString.h
+++ src/OFString.h
@@ -162,10 +162,19 @@
 /**
  * Lower the OFString.
  */
 - lower;
 
+/**
+ * Replaces all occurrences of a string with another string.
+ *
+ * \param str The string to replace
+ * \param repl The string with which it should be replaced
+ */
+- replaceOccurrencesOfString: (OFString*)str
+		  withString: (OFString*)repl;
+
 /**
  * Splits an OFString into an OFArray of OFStrings.
  *
  * \param delimiter The delimiter for splitting
  * \return An autoreleased OFArray with the splitted string

Index: src/OFString.m
==================================================================
--- src/OFString.m
+++ src/OFString.m
@@ -325,10 +325,17 @@
 	@throw [OFNotImplementedException newWithClass: isa
 					   andSelector: _cmd];
 }
 
 - lower
+{
+	@throw [OFNotImplementedException newWithClass: isa
+					   andSelector: _cmd];
+}
+
+- replaceOccurrencesOfString: (OFString*)str
+		  withString: (OFString*)repl
 {
 	@throw [OFNotImplementedException newWithClass: isa
 					   andSelector: _cmd];
 }
 

Index: tests/OFString/OFString.m
==================================================================
--- tests/OFString/OFString.m
+++ tests/OFString/OFString.m
@@ -23,11 +23,11 @@
 #define ZD "%zd"
 #else
 #define ZD "%u"
 #endif
 
-#define NUM_TESTS 25
+#define NUM_TESTS 27
 #define SUCCESS								\
 	printf("\r\033[1;%dmTests successful: " ZD "/%d\033[0m",	\
 	    (i == NUM_TESTS - 1 ? 32 : 33), i + 1, NUM_TESTS);		\
 	fflush(stdout);
 #define FAIL								\
@@ -111,10 +111,19 @@
 
 	CHECK([[@"foo\"ba'_$" urlencode] isEqual: @"foo%22ba%27_%24"])
 	CHECK([[@"foo%20bar%22%24" urldecode] isEqual: @"foo bar\"$"])
 	CHECK_EXCEPT([@"foo%bar" urldecode], OFInvalidEncodingException)
 	CHECK_EXCEPT([@"foo%FFbar" urldecode], OFInvalidEncodingException)
+
+	s1 = [@"asd fo asd fofo asd" mutableCopy];
+	[s1 replaceOccurrencesOfString: @"fo"
+			    withString: @"foo"];
+	CHECK([s1 isEqual: @"asd foo asd foofoo asd"])
+	s1 = [@"XX" mutableCopy];
+	[s1 replaceOccurrencesOfString: @"X"
+			    withString: @"XX"];
+	CHECK([s1 isEqual: @"XXXX"])
 
 	puts("");
 
 	return 0;
 }