diff --git a/string.c b/string.c
index c63f59a..95b13c8 100644
--- a/string.c
+++ b/string.c
@@ -6097,6 +6097,106 @@ 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 void
+line_yield(VALUE str, const char *sub, const char *subend)
+{
+    long len = RSTRING_LEN(str);
+    const char *ptr = RSTRING_PTR(str);
+    VALUE line = rb_str_new5(str, sub, subend - sub);
+
+    OBJ_INFECT(line, str);
+    rb_enc_cr_str_copy_for_substr(line, str);
+    rb_yield(line);
+    str_mod_check(str, ptr, len);
+
+    return;
+}
+
+static void
+str_each_line_valid(VALUE str, VALUE rs, unsigned int newline, rb_encoding *enc)
+{
+    int n, rspara = 0;
+    long index, rslen;
+    const char *ptr, *pend, *sub, *subend;
+    const char *adjusted, *rsptr;
+
+    ptr = sub = subend = RSTRING_PTR(str);
+    pend = RSTRING_END(str);
+
+    rslen = RSTRING_LEN(rs);
+
+    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 (sub < pend) {
+	index = rb_memsearch(rsptr, rslen, sub, pend - sub, enc);
+	if (index < 0) break;
+	subend = sub + index + rslen;
+	if (rspara) {
+	    rb_enc_codepoint_len(subend, pend, &n, enc);
+	    while (subend < pend && rb_enc_codepoint(subend, pend, enc) == newline)
+		subend += n;
+	}
+	adjusted = rb_enc_left_char_head(ptr, subend, pend, enc);
+	if (subend == adjusted) line_yield(str, sub, subend);
+	sub = subend;
+    }
+
+    if (subend < pend) line_yield(str, subend, pend);
+
+    return;
+}
+
+static void
+str_each_line_invalid(VALUE str, VALUE rs, unsigned int newline, rb_encoding *enc)
+{
+    int n;
+    long rslen;
+    const char *sub, *subend, *pend, *rsptr;
+
+    sub = subend = RSTRING_PTR(str);
+    pend = RSTRING_END(str);
+
+    rsptr = RSTRING_PTR(rs);
+    rslen = RSTRING_LEN(rs);
+
+    while (sub < pend) {
+	unsigned int c = rb_enc_codepoint_len(sub, pend, &n, enc);
+
+      again:
+	if (rslen == 0 && c == newline) {
+	    subend += n;
+	    if (subend < pend && (c = rb_enc_codepoint_len(subend, pend, &n, enc)) != newline) {
+		goto again;
+	    }
+	    while (subend < pend && rb_enc_codepoint(subend, pend, enc) == newline) {
+		subend += n;
+	    }
+	    subend -= n;
+	}
+	if (c == newline &&
+	    (rslen <= 1 ||
+	    (pend - subend >= rslen && memcmp(rsptr, subend, rslen) == 0))) {
+	    subend += rslen ? rslen : n;
+	    line_yield(str, sub, subend);
+	    sub = subend;
+	}
+	subend += n;
+    }
+
+    if (subend < pend) line_yield(str, subend, pend);
+
+    return;
+}
 
 /*
  *  call-seq:
@@ -6141,95 +6241,40 @@ rb_str_each_line(int argc, VALUE *argv, VALUE str)
     rb_encoding *enc;
     VALUE rs;
     unsigned int newline;
-    const char *p, *pend, *s, *ptr;
-    long len, rslen;
-    VALUE line;
-    int n;
     VALUE orig = str;
 
-    if (argc == 0) {
+    if (argc == 0)
 	rs = rb_rs;
-    }
-    else {
+    else
 	rb_scan_args(argc, argv, "01", &rs);
-    }
+
     RETURN_ENUMERATOR(str, argc, argv);
+
     if (NIL_P(rs)) {
 	rb_yield(str);
 	return orig;
     }
-    str = rb_str_new4(str);
-    ptr = p = s = RSTRING_PTR(str);
-    pend = p + RSTRING_LEN(str);
-    len = RSTRING_LEN(str);
     StringValue(rs);
-    if (rs == rb_default_rs) {
-	enc = rb_enc_get(str);
-	while (p < pend) {
-	    char *p0;
 
-	    p = memchr(p, '\n', pend - p);
-	    if (!p) break;
-	    p0 = rb_enc_left_char_head(s, p, pend, enc);
-	    if (!rb_enc_is_newline(p0, pend, enc)) {
-		p++;
-		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);
-	    str_mod_check(str, ptr, len);
-	    s = p;
-	}
-	goto finish;
-    }
+    str = rb_str_new4(str);
 
-    enc = rb_enc_check(str, rs);
-    rslen = RSTRING_LEN(rs);
-    if (rslen == 0) {
-	newline = '\n';
+    if (rs == rb_rs) {
+	enc = rb_enc_get(str);
+	rs = rb_str_encode(rb_rs, rb_enc_from_encoding(enc), 0, Qnil);
     }
     else {
-	newline = rb_enc_codepoint(RSTRING_PTR(rs), RSTRING_END(rs), enc);
+	enc = rb_enc_check(str, rs);
     }
 
-    while (p < pend) {
-	unsigned int c = rb_enc_codepoint_len(p, pend, &n, enc);
-
-      again:
-	if (rslen == 0 && c == newline) {
-	    p += n;
-	    if (p < pend && (c = rb_enc_codepoint_len(p, pend, &n, enc)) != newline) {
-		goto again;
-	    }
-	    while (p < pend && rb_enc_codepoint(p, pend, enc) == newline) {
-		p += n;
-	    }
-	    p -= n;
-	}
-	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);
-	    str_mod_check(str, ptr, len);
-	    s = p + (rslen ? rslen : n);
-	}
-	p += n;
-    }
+    if (RSTRING_LEN(rs) == 0)
+	newline = '\n';
+    else
+	newline = rb_enc_codepoint(RSTRING_PTR(rs), RSTRING_END(rs), enc);
 
-  finish:
-    if (s != pend) {
-	line = rb_str_new5(str, s, pend - s);
-	OBJ_INFECT(line, str);
-	rb_enc_cr_str_copy_for_substr(line, str);
-	rb_yield(line);
-	RB_GC_GUARD(str);
-    }
+    if (rb_str_valid_encoding_p(str) && rb_str_valid_encoding_p(rs))
+	str_each_line_valid(str, rs, newline, enc);
+    else
+	str_each_line_invalid(str, rs, newline, enc);
 
     return orig;
 }