Treat backslash as normal char in TextElements

spec/CHANGELOG.md

@@ -14,6 +14,16 @@
     consistent with how `Identifiers` of variables don't include the `$`
     sigil either.
 
+  - Treat backslash (`\`) as a regular character in `TextElements`. (#123)
+
+    Backslash does no longer have special escaping powers when used in
+    `TextElements`. It's still recognized as special in `StringLiterals`,
+    however. `StringLiterals` can be used to insert all special-purpose
+    characters in text. For instance, `{"{"}` will insert the literal opening
+    curly brace (`{`), `{"\u00A0"}` will insert the non-breaking space, and
+    `{"   "}` can be used to make a translation start or end with whitespace,
+    which would otherwise by trimmed by `Pattern.`
+
 ## 0.7.0 (October 15, 2018)
 
   - Relax the indentation requirement. (#87)

spec/fluent.ebnf

@@ -57,7 +57,7 @@ InlineExpression    ::= StringLiteral
                       | inline_placeable
 
 /* Literals */
-StringLiteral       ::= quote quoted_text_char* quote
+StringLiteral       ::= "\"" quoted_char* "\""
 NumberLiteral       ::= "-"? digit+ ("." digit+)?
 
 /* Inline Expressions */
@@ -84,22 +84,26 @@ Identifier          ::= [a-zA-Z] [a-zA-Z0-9_-]*
 Function            ::= [A-Z] [A-Z_?-]*
 
 /* Characters */
-backslash           ::= "\\"
-quote               ::= "\""
-/* Any Unicode character from BMP excluding C0 control characters, space,
+/* Any Unicode character excluding C0 control characters (but including tab),
  * surrogate blocks and non-characters (U+FFFE, U+FFFF).
- * Cf. https://www.w3.org/TR/REC-xml/#NT-Char
- */
-regular_char        ::= [\\u{21}-\\u{D7FF}\\u{E000}-\\u{FFFD}\\u{10000}-\\u{10FFFF}]
-text_char           ::= blank_inline
-                      | "\u0009"
-                      | /\\u[0-9a-fA-F]{4}/
-                      | (backslash backslash)
-                      | (backslash "{")
-                      | (regular_char - "{" - backslash)
+ * Cf. https://www.w3.org/TR/REC-xml/#NT-Char */
+any_char            ::= [\\u{9}\\u{20}-\\u{D7FF}\\u{E000}-\\u{FFFD}]
+                      | [\\u{10000}-\\u{10FFFF}]
+/* The opening brace in text starts a placeable. */
+special_text_char   ::= "{"
+/* Double quote and backslash need to be escaped in string literals. */
+special_quoted_char ::= "\""
+                      | "\\"
+text_char           ::= any_char - special_text_char
+/* Indented text may not start with characters which mark its end. */
 indented_char       ::= text_char - "}" - "[" - "*" - "."
-quoted_text_char    ::= (text_char - quote)
-                      | (backslash quote)
+special_escape      ::= "\\" special_quoted_char
+unicode_escape      ::= "\\u" /[0-9a-fA-F]{4}/
+/* The literal opening brace { is allowed in string literals because they may
+ * not have placeables. */
+quoted_char         ::= (any_char - special_quoted_char)
+                      | special_escape
+                      | unicode_escape
 digit               ::= [0-9]
 
 /* Whitespace */

syntax/grammar.mjs

@@ -199,9 +199,9 @@ let InlineExpression = defer(() =>
 /* Literals */
 let StringLiteral = defer(() =>
     sequence(
-        quote,
-        repeat(quoted_text_char),
-        quote)
+        string("\""),
+        repeat(quoted_char),
+        string("\""))
     .map(element_at(1))
     .map(join)
     .chain(into(FTL.StringLiteral)));
@@ -373,48 +373,59 @@ let Function =
 /* ---------- */
 /* Characters */
 
-let backslash = string("\\");
-let quote = string("\"");
-
-/* Any Unicode character from BMP excluding C0 control characters, space,
+/* Any Unicode character excluding C0 control characters (but including tab),
  * surrogate blocks and non-characters (U+FFFE, U+FFFF).
- * Cf. https://www.w3.org/TR/REC-xml/#NT-Char
- */
-let regular_char =
-    charset("\\u{21}-\\u{D7FF}\\u{E000}-\\u{FFFD}\\u{10000}-\\u{10FFFF}");
+ * Cf. https://www.w3.org/TR/REC-xml/#NT-Char */
+let any_char =
+    either(
+        charset("\\u{9}\\u{20}-\\u{D7FF}\\u{E000}-\\u{FFFD}"),
+        charset("\\u{10000}-\\u{10FFFF}"));
 
-let text_char = defer(() =>
+/* The opening brace in text starts a placeable. */
+let special_text_char =
+    string("{");
+
+/* Double quote and backslash need to be escaped in string literals. */
+let special_quoted_char =
     either(
-        blank_inline,
-        string("\u0009"),
-        regex(/\\u[0-9a-fA-F]{4}/),
-        sequence(
-            backslash,
-            backslash).map(join),
-        sequence(
-            backslash,
-            string("{")).map(join),
-        and(
-            not(backslash),
-            not(string("{")),
-            regular_char)));
+        string("\""),
+        string("\\"));
+
+let text_char =
+    and(
+        not(special_text_char),
+        any_char);
 
-let indented_char = defer(() =>
+/* Indented text may not start with characters which mark its end. */
+let indented_char =
     and(
         not(string(".")),
         not(string("*")),
         not(string("[")),
         not(string("}")),
-        text_char));
+        text_char);
+
+let special_escape =
+    sequence(
+        string("\\"),
+        special_quoted_char)
+    .map(join);
 
-let quoted_text_char =
+let unicode_escape =
+    sequence(
+        string("\\u"),
+        regex(/[0-9a-fA-F]{4}/))
+    .map(join);
+
+/* The literal opening brace { is allowed in string literals because they may
+ * not have placeables. */
+let quoted_char =
     either(
         and(
-            not(quote),
-            text_char),
-        sequence(
-            backslash,
-            quote).map(join));
+            not(special_quoted_char),
+            any_char),
+        special_escape,
+        unicode_escape);
 
 let digit = charset("0-9");
 

test/fixtures/escaped_characters.ftl

@@ -1,9 +1,22 @@
-backslash = Value with \\ (an escaped backslash)
-closing-brace = Value with \{ (a closing brace)
-unicode-escape = \u0041
-escaped-unicode = \\u0041
+## Literal text
+text-backslash-one = Value with \ a backslash
+text-backslash-two = Value with \\ two backslashes
+text-backslash-brace = Value with \{placeable}
+text-backslash-u = \u0041
+text-backslash-backslash-u = \\u0041
 
-## String Expressions
+## String literals
 quote-in-string = {"\""}
 backslash-in-string = {"\\"}
+# ERROR Mismatched quote
 mismatched-quote = {"\\""}
+# ERROR Unknown escape
+unknown-escape = {"\x"}
+
+## Unicode escapes
+string-unicode-sequence = {"\u0041"}
+string-escaped-unicode = {"\\u0041"}
+
+## Literal braces
+brace-open = An opening {"{"} brace.
+brace-close = A closing } brace.