diff --git a/configure.in b/configure.in
index 9a679de..4bba5eb 100644
--- a/configure.in
+++ b/configure.in
@@ -1426,7 +1426,7 @@ AC_CHECK_FUNCS(fmod killpg wait4 waitpid fork spawnv syscall __syscall chroot ge
 	      setuid setgid daemon select_large_fdset setenv unsetenv\
               mktime timegm gmtime_r clock_gettime gettimeofday poll ppoll\
               pread sendfile shutdown sigaltstack dl_iterate_phdr\
-              dup3 pipe2 posix_memalign memalign)
+              dup3 pipe2 posix_memalign memalign memmem)
 
 AC_CACHE_CHECK(for unsetenv returns a value, rb_cv_unsetenv_return_value,
   [AC_TRY_COMPILE([
@@ -2726,6 +2726,39 @@ if test "${universal_binary-no}" = yes ; then
     AC_MSG_ERROR([failed])
     ])])
 fi
+AC_CACHE_CHECK(for broken memmem, rb_cv_broken_memmem,
+  [AC_TRY_RUN([
+#include <string.h>
+
+int
+main()
+{
+  char *str = "hogefugafoobar";
+  char *rs = "foo";
+  char *empty = "";
+  char *p;
+
+  p = memmem(str, strlen(str), rs, strlen(rs));
+  if (p == str+8) {
+    p = memmem(str, strlen(str), empty, strlen(empty));
+    if (p == str)
+      return 0;
+    else
+      return 1;
+  }
+  else {
+    return 1;
+  }
+}
+  ],
+  rb_cv_broken_memmem=no,
+  rb_cv_broken_memmem=yes,
+  rb_cv_broken_memmem=yes)
+])
+
+if test "$rb_cv_broken_memmem" = yes; then
+  AC_DEFINE(BROKEN_MEMMEM, 1)
+fi
 
 CPPFLAGS="$CPPFLAGS "'$(DEFS)'
 test -z "$CPPFLAGS" || CPPFLAGS="$CPPFLAGS "; CPPFLAGS="$CPPFLAGS"'${cppflags}'
diff --git a/string.c b/string.c
index d06a996..9c9cebd 100644
--- a/string.c
+++ b/string.c
@@ -5967,6 +5967,17 @@ rb_str_split(VALUE str, const char *sep0)
     return rb_str_split_m(1, &sep, str);
 }
 
+static VALUE rb_str_valid_encoding_p(VALUE str);
+
+static const char *line_yield(VALUE str, const char *substr, long sublen){
+    VALUE line = rb_str_new5(str, substr, sublen);
+
+    OBJ_INFECT(line, str);
+    rb_enc_cr_str_copy_for_substr(line, str);
+    rb_yield(line);
+
+    return substr + sublen;
+}
 
 /*
  *  call-seq:
@@ -6046,10 +6057,7 @@ rb_str_each_line(int argc, VALUE *argv, VALUE str)
 		continue;
 	    }
 	    p = p0 + rb_enc_mbclen(p0, pend, enc);
-	    line = rb_str_new5(str, s, p - s);
-	    OBJ_INFECT(line, str);
-	    rb_enc_cr_str_copy_for_substr(line, str);
-	    rb_yield(line);
+	    line_yield(str, s, p - s);
 	    str_mod_check(str, ptr, len);
 	    s = p;
 	}
@@ -6058,12 +6066,50 @@ rb_str_each_line(int argc, VALUE *argv, VALUE str)
 
     enc = rb_enc_check(str, rs);
     rslen = RSTRING_LEN(rs);
-    if (rslen == 0) {
+
+    if (rslen == 0)
 	newline = '\n';
+
+#if defined(HAVE_MEMMEM) && !defined(BROKEN_MEMMEM)
+    if (rb_str_valid_encoding_p(str) && rb_str_valid_encoding_p(rs)) {
+	int rspara = 0;
+	long sublen, rest = len;
+	const char *e, *pp, *rsptr, *ss = s;
+
+	if (rslen == 0) {
+	    rspara = 1;
+	    rs = rb_usascii_str_new("\n\n", 2);
+	    if (!rb_enc_asciicompat(enc))
+		rs = rb_str_encode(rs, rb_enc_from_encoding(enc), 0, Qnil);
+	    rslen = RSTRING_LEN(rs);
+	}
+
+	rsptr = RSTRING_PTR(rs);
+
+	while (e = memmem(ss, rest, rsptr, rslen)) {
+	    if (rspara) {
+		p = e;
+		rb_enc_codepoint_len(p, pend, &n, enc);
+		while (p < pend && rb_enc_codepoint(p, pend, enc) == newline)
+		    p += n;
+		sublen = p-ss;
+	    }
+	    else {
+		sublen = e-ss+rslen;
+	    }
+	    pp = rb_enc_left_char_head(ss, e, ss+rest, enc);
+	    if (pp == e) {
+		s = line_yield(str, s, sublen);
+		str_mod_check(str, ptr, len);
+	    }
+	    ss += sublen;
+	    rest -= sublen;
+	}
+	goto finish;
     }
-    else {
+#endif
+    if (rslen != 0)
 	newline = rb_enc_codepoint(RSTRING_PTR(rs), RSTRING_END(rs), enc);
-    }
 
     while (p < pend) {
 	unsigned int c = rb_enc_codepoint_len(p, pend, &n, enc);
@@ -6082,12 +6128,8 @@ rb_str_each_line(int argc, VALUE *argv, VALUE str)
 	if (c == newline &&
 	    (rslen <= 1 ||
 	     (pend - p >= rslen && memcmp(RSTRING_PTR(rs), p, rslen) == 0))) {
-	    line = rb_str_new5(str, s, p - s + (rslen ? rslen : n));
-	    OBJ_INFECT(line, str);
-	    rb_enc_cr_str_copy_for_substr(line, str);
-	    rb_yield(line);
+	    s = line_yield(str, s, p - s + (rslen ? rslen : n));
 	    str_mod_check(str, ptr, len);
-	    s = p + (rslen ? rslen : n);
 	}
 	p += n;
     }