Keep hex escapes in string literals (fsprojects#1508) (fsprojects#1509)

* Keep hex escapes in string literals (fsprojects#1508)

* Add more patterns to escapedCharacterRegex (fsprojects#1508)

src/Fantomas.Tests/SynConstTests.fs

@@ -294,3 +294,17 @@ let f (c: char) =
     | '\u0000'
     | _ -> ()
 """
+
+[<Test>]
+let ``hex escape in string literal should be preserved, 1508`` () =
+    formatSourceString
+        false
+        """let hexEscape = "\x00"
+let controlEscapes = "\a \b \f \v"
+"""
+        config
+    |> should
+        equal
+        """let hexEscape = "\x00"
+let controlEscapes = "\a \b \f \v"
+"""

src/Fantomas/TokenParser.fs

@@ -507,7 +507,7 @@ let private (|InterpStringEndOrPartToken|_|) token =
         None
 
 let escapedCharacterRegex =
-    System.Text.RegularExpressions.Regex("(\\\\(n|r|u|'|\\\"|\\\\))+")
+    System.Text.RegularExpressions.Regex("(\\\\(a|b|f|n|r|u|v|x|'|\\\"|\\\\))+")
 
 let rec private (|EndOfInterpolatedString|_|) tokens =
     match tokens with