regexp/syntax: fix factoring of common prefixes in alternations

In the past, `a.*?c|a.*?b` was factored to `a.*?[bc]`. Thus, given
"abc" as its input string, the automaton would consume "ab" and
then stop (when unanchored) whereas it should consume all of "abc"
as per leftmost semantics.

Fixes #13812.

Change-Id: I67ac0a353d7793b3d0c9c4aaf22d157621dfe784
Reviewed-on: https://go-review.googlesource.com/18357Reviewed-by: Russ Cox <rsc@golang.org>

src/regexp/syntax/parse.go

@@ -470,9 +470,14 @@ func (p *parser) factor(sub []*Regexp, flags Flags) []*Regexp {
 	}
 	sub = out
 
-	// Round 2: Factor out common complex prefixes,
-	// just the first piece of each concatenation,
-	// whatever it is.  This is good enough a lot of the time.
+	// Round 2: Factor out common simple prefixes,
+	// just the first piece of each concatenation.
+	// This will be good enough a lot of the time.
+	//
+	// Complex subexpressions (e.g. involving quantifiers)
+	// are not safe to factor because that collapses their
+	// distinct paths through the automaton, which affects
+	// correctness in some cases.
 	start = 0
 	out = sub[:0]
 	var first *Regexp
@@ -485,7 +490,9 @@ func (p *parser) factor(sub []*Regexp, flags Flags) []*Regexp {
 		var ifirst *Regexp
 		if i < len(sub) {
 			ifirst = p.leadingRegexp(sub[i])
-			if first != nil && first.Equal(ifirst) {
+			if first != nil && first.Equal(ifirst) &&
+				// first must be a character class OR a fixed repeat of a character class.
+				(isCharClass(first) || (first.Op == OpRepeat && first.Min == first.Max && isCharClass(first.Sub[0]))) {
 				continue
 			}
 		}

src/regexp/syntax/parse_test.go

@@ -172,7 +172,7 @@ var parseTests = []parseTest{
 
 	// Factoring.
 	{`abc|abd|aef|bcx|bcy`, `alt{cat{lit{a}alt{cat{lit{b}cc{0x63-0x64}}str{ef}}}cat{str{bc}cc{0x78-0x79}}}`},
-	{`ax+y|ax+z|ay+w`, `cat{lit{a}alt{cat{plus{lit{x}}cc{0x79-0x7a}}cat{plus{lit{y}}lit{w}}}}`},
+	{`ax+y|ax+z|ay+w`, `cat{lit{a}alt{cat{plus{lit{x}}lit{y}}cat{plus{lit{x}}lit{z}}cat{plus{lit{y}}lit{w}}}}`},
 
 	// Bug fixes.
 	{`(?:.)`, `dot{}`},
@@ -195,12 +195,13 @@ var parseTests = []parseTest{
 	{`abc|x|abd`, `alt{str{abc}lit{x}str{abd}}`},
 	{`(?i)abc|ABD`, `cat{strfold{AB}cc{0x43-0x44 0x63-0x64}}`},
 	{`[ab]c|[ab]d`, `cat{cc{0x61-0x62}cc{0x63-0x64}}`},
-	{`(?:xx|yy)c|(?:xx|yy)d`,
-		`cat{alt{str{xx}str{yy}}cc{0x63-0x64}}`},
+	{`.c|.d`, `cat{dot{}cc{0x63-0x64}}`},
 	{`x{2}|x{2}[0-9]`,
 		`cat{rep{2,2 lit{x}}alt{emp{}cc{0x30-0x39}}}`},
 	{`x{2}y|x{2}[0-9]y`,
 		`cat{rep{2,2 lit{x}}alt{lit{y}cat{cc{0x30-0x39}lit{y}}}}`},
+	{`a.*?c|a.*?b`,
+		`cat{lit{a}alt{cat{nstar{dot{}}lit{c}}cat{nstar{dot{}}lit{b}}}}`},
 
 	// Valid repetitions.
 	{`((((((((((x{2}){2}){2}){2}){2}){2}){2}){2}){2}))`, ``},

src/regexp/testdata/re2-search.txt

@@ -3665,3 +3665,8 @@ regexps
 "(?:a\\C*|ba\\C)$"
 -;-;-;-
 -;1-4;-;1-4
+strings
+"abc"
+regexps
+"a.*?c|a.*?b"
+0-3;0-3;0-3;0-3