fixes #44 change RE2 option to allow default character escape sequences

Author: dlclark

README.md

@@ -81,6 +81,7 @@ The default behavior of `regexp2` is to match the .NET regexp engine, however th
 * add support for python-style capture groups (e.g. `(P<name>re)`)
 * change singleline behavior for `$` to only match end of string (like RE2) (see [#24](https://github.com/dlclark/regexp2/issues/24))
 * change the character classes `\d` `\s` and `\w` to match the same characters as RE2. NOTE: if you also use the `ECMAScript` option then this will change the `\s` character class to match ECMAScript instead of RE2.  ECMAScript allows more whitespace characters in `\s` than RE2 (but still fewer than the the default behavior).
+* allow character escape sequences to have defaults. For example, by default `\_` isn't a known character escape and will fail to compile, but in RE2 mode it will match the literal character `_`
 
 ```go
 re := regexp2.MustCompile(`Your RE2-compatible pattern`, regexp2.RE2)

regexp_re2_test.go

@@ -194,3 +194,30 @@ func TestRegularSpace(t *testing.T) {
 		t.Fatal("Expected no match")
 	}
 }
+
+func TestEscapeLiteralDefaults(t *testing.T) {
+	_, err := Compile(`a\_test`, 0)
+	if err == nil {
+		t.Fatal("Expected compile fail")
+	}
+
+	r := MustCompile(`a\_test`, RE2)
+	if m, _ := r.MatchString("a_test"); !m {
+		t.Fatal("Expected match")
+	}
+	if m, _ := r.MatchString("a\\_test"); m {
+		t.Fatal("Expected no match")
+	}
+}
+
+/*
+func TestRE2EndZ_Singleline(t *testing.T) {
+	// PCRE allows for \n after the $ and RE2 doesn't
+	r := MustCompile(`^ac$\Z`, RE2|Debug)
+	if m, _ := r.MatchString("ac"); m {
+		t.Fatal("Expected no match")
+	}
+	if m, _ := r.MatchString("ac\n"); m {
+		t.Fatal("Expected no match")
+	}
+}*/

syntax/parser.go

@@ -1715,7 +1715,7 @@ func (p *parser) scanCharEscape() (r rune, err error) {
 	case 'c':
 		r, err = p.scanControl()
 	default:
-		if !p.useOptionE() && IsWordChar(ch) {
+		if !p.useOptionE() && !p.useRE2() && IsWordChar(ch) {
 			return 0, p.getErr(ErrUnrecognizedEscape, string(ch))
 		}
 		return ch, nil