Fix escaping

README.md

@@ -189,6 +189,21 @@ tr.Trans(
 )
 ```
 
+### Escaping
+
+Sometimes you need to print `{`, `'`, or `#`. You could escape them with `'` char.
+
+```yaml
+# translations/messages.en.yaml
+escape: "'{foo} is ''{foo}''"
+```
+
+```go
+tr.Trans("escape", mf.Arg("foo", "bar"))
+// {foo} is 'bar'
+```
+
+
 ## MessageFormat overview
 
 ### Placeholders

message/build.go

@@ -32,6 +32,10 @@ func Build(in parse.Message, lang language.Tag) (Evalable, error) {
 }
 
 func buildFragment(f parse.Fragment, lang language.Tag) (Evalable, error) {
+	if len(f.Escaped) > 0 {
+		return Content(f.Escaped[1:]), nil
+	}
+
 	if len(f.Text) > 0 {
 		return Content(f.Text), nil
 	}

mf/translator_test.go

@@ -418,6 +418,54 @@ func Test_translator_Trans(t *testing.T) {
 			"First step on the Moon on July 21, 1969.",
 			false,
 		},
+		{
+			"escaping quotes not required",
+			"foo ' {foo}.",
+			language.English,
+			[]TranslationArg{Arg("foo", "bar")},
+			"foo ' bar.",
+			false,
+		},
+		{
+			"escaping single curly brace",
+			"foo '{ {foo}.",
+			language.English,
+			[]TranslationArg{Arg("foo", "bar")},
+			"foo { bar.",
+			false,
+		},
+		{
+			"escaping single quote",
+			"foo '' {foo}.",
+			language.English,
+			[]TranslationArg{Arg("foo", "bar")},
+			"foo ' bar.",
+			false,
+		},
+		{
+			"escaping single quote before curly brace",
+			"foo ''{foo}''.",
+			language.English,
+			[]TranslationArg{Arg("foo", "bar")},
+			"foo 'bar'.",
+			false,
+		},
+		{
+			"escaping",
+			"foo '{ ''{foo} {num, plural, one {''#'' '# ' '{ one}, other {other}}.",
+			language.English,
+			[]TranslationArg{Arg("foo", "bar"), Arg("num", 1)},
+			"foo { 'bar '1' # ' { one.",
+			false,
+		},
+		{
+			"escaping",
+			"'{foo} is ''{foo}''",
+			language.English,
+			[]TranslationArg{Arg("foo", "bar")},
+			"{foo} is 'bar'",
+			false,
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {

parse/parser.go

@@ -32,7 +32,8 @@ type Message struct {
 }
 
 type Fragment struct {
-	Text       string    `(@String | @SubMessageString)`
+	Escaped    string    `(@Escaped | @SubEscaped)`
+	Text       string    `| (@String | @SubMessageString | @Quote | @SubQuote)`
 	PlainArg   *PlainArg `| @@`
 	Func       *Func     `| @@`
 	Expr       *Expr     `| @@`
@@ -42,7 +43,9 @@ type Fragment struct {
 func NewParser() *participle.Parser[Message] {
 	def := lexer.MustStateful(lexer.Rules{
 		"Root": {
-			{Name: `String`, Pattern: `[^{]+`, Action: nil},
+			{Name: `Escaped`, Pattern: `'{|''`, Action: nil},
+			{Name: `Quote`, Pattern: `'`, Action: nil},
+			{Name: `String`, Pattern: `[^'{]+`, Action: nil},
 			{Name: `Expr`, Pattern: `{`, Action: lexer.Push("Expr")},
 		},
 		"Expr": {
@@ -55,8 +58,10 @@ func NewParser() *participle.Parser[Message] {
 			{Name: `SubMessage`, Pattern: `{`, Action: lexer.Push("SubMessage")},
 		},
 		"SubMessage": {
-			{Name: `SubMessageString`, Pattern: `[^{^}^#]+`, Action: nil},
+			{Name: `SubEscaped`, Pattern: `'{|''|'#|'}`, Action: nil},
+			{Name: `SubQuote`, Pattern: `'`, Action: nil},
 			{Name: `Octothorpe`, Pattern: `#`, Action: nil},
+			{Name: `SubMessageString`, Pattern: `[^{^}^#^']+`, Action: nil},
 			{Name: `Expr`, Pattern: `{`, Action: lexer.Push("Expr")},
 			{Name: `SubMessageEnd`, Pattern: `}`, Action: lexer.Pop()},
 		},

parse/parser_test.go

@@ -0,0 +1,26 @@
+package parse
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestParser_Escaping(t *testing.T) {
+	parser := NewParser()
+
+	msg, err := parser.Parse("", strings.NewReader("foo '{ '' ' foo"))
+
+	require.NoError(t, err)
+	assert.Len(t, msg.Fragments, 7)
+
+	assert.Equal(t, &Fragment{Text: "foo "}, msg.Fragments[0])
+	assert.Equal(t, &Fragment{Escaped: "'{"}, msg.Fragments[1])
+	assert.Equal(t, &Fragment{Text: " "}, msg.Fragments[2])
+	assert.Equal(t, &Fragment{Escaped: "''"}, msg.Fragments[3])
+	assert.Equal(t, &Fragment{Text: " "}, msg.Fragments[4])
+	assert.Equal(t, &Fragment{Text: "'"}, msg.Fragments[5])
+	assert.Equal(t, &Fragment{Text: " foo"}, msg.Fragments[6])
+}