diff --git a/src/pkg/exp/regexp/exec.go b/src/pkg/exp/regexp/exec.go
index 88b16032ee6..43499a92f02 100644
--- a/src/pkg/exp/regexp/exec.go
+++ b/src/pkg/exp/regexp/exec.go
@@ -128,6 +128,11 @@ func (m *machine) match(i input, pos int) bool {
 		if width == 0 {
 			break
 		}
+		if len(m.matchcap) == 0 && m.matched {
+			// Found a match and not paying attention
+			// to where it is, so any match will do.
+			break
+		}
 		pos += width
 		rune, width = rune1, width1
 		if rune != endOfText {
@@ -155,37 +160,37 @@ func (m *machine) clear(q *queue) {
 // which starts at position pos and ends at nextPos.
 // nextCond gives the setting for the empty-width flags after c.
 func (m *machine) step(runq, nextq *queue, pos, nextPos, c int, nextCond syntax.EmptyOp) {
+	longest := m.re.longest
 	for j := 0; j < len(runq.dense); j++ {
 		d := &runq.dense[j]
 		t := d.t
 		if t == nil {
 			continue
 		}
-		/*
-			 * If we support leftmost-longest matching:
-				if longest && matched && match[0] < t.cap[0] {
-					m.free(t)
-					continue
-				}
-		*/
-
+		if longest && m.matched && len(t.cap) > 0 && m.matchcap[0] < t.cap[0] {
+			m.free(t)
+			continue
+		}
 		i := t.inst
 		switch i.Op {
 		default:
 			panic("bad inst")
 
 		case syntax.InstMatch:
-			if len(t.cap) > 0 {
+			if len(t.cap) > 0 && (!longest || !m.matched || m.matchcap[1] < pos) {
 				t.cap[1] = pos
 				copy(m.matchcap, t.cap)
 			}
-			m.matched = true
-			for _, d := range runq.dense[j+1:] {
-				if d.t != nil {
-					m.free(d.t)
+			if !longest {
+				// First-match mode: cut off all lower-priority threads.
+				for _, d := range runq.dense[j+1:] {
+					if d.t != nil {
+						m.free(d.t)
+					}
 				}
+				runq.dense = runq.dense[:0]
 			}
-			runq.dense = runq.dense[:0]
+			m.matched = true
 
 		case syntax.InstRune:
 			if i.MatchRune(c) {
diff --git a/src/pkg/exp/regexp/exec_test.go b/src/pkg/exp/regexp/exec_test.go
index 15c4c532a40..69f673aca9a 100644
--- a/src/pkg/exp/regexp/exec_test.go
+++ b/src/pkg/exp/regexp/exec_test.go
@@ -164,29 +164,29 @@ func TestRE2(t *testing.T) {
 				continue
 			}
 			res := strings.Split(line, ";")
-			if len(res) != 2 {
-				t.Fatalf("re2.txt:%d: have %d test results, want 2", lineno, len(res))
+			if len(res) != len(run) {
+				t.Fatalf("re2.txt:%d: have %d test results, want %d", lineno, len(res), len(run))
 			}
-			// res[0] is full match
-			// res[1] is partial match
-			// Run partial match first; don't bother with full if partial fails.
-			have := re.FindStringSubmatchIndex(text)
-			want := parseResult(t, lineno, res[1])
-			if !same(have, want) {
-				t.Errorf("re2.txt:%d: %#q.FindSubmatchIndex(%#q) = %v, want %v", lineno, re, text, have, want)
-				if nfail++; nfail >= 100 {
-					t.Fatalf("stopping after %d errors", nfail)
+			for i := range res {
+				have, suffix := run[i](re, refull, text)
+				want := parseResult(t, lineno, res[i])
+				if !same(have, want) {
+					t.Errorf("re2.txt:%d: %#q%s.FindSubmatchIndex(%#q) = %v, want %v", lineno, re, suffix, text, have, want)
+					if nfail++; nfail >= 100 {
+						t.Fatalf("stopping after %d errors", nfail)
+					}
+					continue
 				}
-				continue
-			}
-			have = refull.FindStringSubmatchIndex(text)
-			want = parseResult(t, lineno, res[0])
-			if !same(have, want) {
-				t.Errorf("re2.txt:%d: %#q.FindSubmatchIndex(%#q) = %v, want %v", lineno, refull, text, have, want)
-				if nfail++; nfail >= 100 {
-					t.Fatalf("stopping after %d errors", nfail)
+				b, suffix := match[i](re, refull, text)
+				if b != (want != nil) {
+					t.Errorf("re2.txt:%d: %#q%s.MatchString(%#q) = %v, want %v", lineno, re, suffix, text, b, !b)
+					if nfail++; nfail >= 100 {
+						t.Fatalf("stopping after %d errors", nfail)
+					}
+					continue
 				}
 			}
+
 		default:
 			t.Fatalf("re2.txt:%d: out of sync: %s\n", lineno, line)
 		}
@@ -197,6 +197,60 @@ func TestRE2(t *testing.T) {
 	t.Logf("%d cases tested", ncase)
 }
 
+var run = []func(*Regexp, *Regexp, string) ([]int, string){
+	runFull,
+	runPartial,
+	runFullLongest,
+	runPartialLongest,
+}
+
+func runFull(re, refull *Regexp, text string) ([]int, string) {
+	refull.longest = false
+	return refull.FindStringSubmatchIndex(text), "[full]"
+}
+
+func runPartial(re, refull *Regexp, text string) ([]int, string) {
+	re.longest = false
+	return re.FindStringSubmatchIndex(text), ""
+}
+
+func runFullLongest(re, refull *Regexp, text string) ([]int, string) {
+	refull.longest = true
+	return refull.FindStringSubmatchIndex(text), "[full,longest]"
+}
+
+func runPartialLongest(re, refull *Regexp, text string) ([]int, string) {
+	re.longest = true
+	return re.FindStringSubmatchIndex(text), "[longest]"
+}
+
+var match = []func(*Regexp, *Regexp, string) (bool, string){
+	matchFull,
+	matchPartial,
+	matchFullLongest,
+	matchPartialLongest,
+}
+
+func matchFull(re, refull *Regexp, text string) (bool, string) {
+	refull.longest = false
+	return refull.MatchString(text), "[full]"
+}
+
+func matchPartial(re, refull *Regexp, text string) (bool, string) {
+	re.longest = false
+	return re.MatchString(text), ""
+}
+
+func matchFullLongest(re, refull *Regexp, text string) (bool, string) {
+	refull.longest = true
+	return refull.MatchString(text), "[full,longest]"
+}
+
+func matchPartialLongest(re, refull *Regexp, text string) (bool, string) {
+	re.longest = true
+	return re.MatchString(text), "[longest]"
+}
+
 func isSingleBytes(s string) bool {
 	for _, c := range s {
 		if c >= utf8.RuneSelf {
diff --git a/src/pkg/exp/regexp/re2.txt.gz b/src/pkg/exp/regexp/re2.txt.gz
index 2b8c832e522..85b8eadf860 100644
Binary files a/src/pkg/exp/regexp/re2.txt.gz and b/src/pkg/exp/regexp/re2.txt.gz differ
diff --git a/src/pkg/exp/regexp/regexp.go b/src/pkg/exp/regexp/regexp.go
index 11feecd55e9..1d0fc9df8d4 100644
--- a/src/pkg/exp/regexp/regexp.go
+++ b/src/pkg/exp/regexp/regexp.go
@@ -85,6 +85,7 @@ type Regexp struct {
 	prefixRune     int            // first rune in prefix
 	cond           syntax.EmptyOp // empty-width conditions required at start of match
 	numSubexp      int
+	longest        bool
 
 	// cache of machines for running regexp
 	mu      sync.Mutex