Fix #250, strings vs expressions boundary cases

grammars/arm-expression-string.tmLanguage.json

@@ -6,6 +6,8 @@
 	"$preprocessComment2": "The 'builtin' key/value are special - they're created during build from our function metadata",
 	"$preprocessComment3": "Currently you need to run 'build grammars' after changing this file",
 	"preprocess": {
+		"escaped-json-character": "(?:\\\\(?:[\"\\\\/bfnrt]|u[0-9a-fA-F]{4}))",
+		"escaped-apostrophe": "(?:[']['])",
 		"idchar": "[_$[:alnum:]]",
 		"id": "(?:[_$[:alpha:]]{{idchar}}*)",
 		"ns-userfunc": "(?:({{id}})\\s*(\\.)\\s*({{id}}))",
@@ -42,8 +44,9 @@
 	"repository": {
 		"expressionstring": {
 			"name": "meta.expression.tle.arm",
-			"begin": "\"\\[(?!\\[)",
-			"$beginComment": "A string that starts with [[ is not an expression",
+			"$begin": "(?x) \"\\[(?!\\[) (?= .*) \\]\"",
+			"begin": "(?x)  \"\\[(?!\\[)  (?= ([^\"]|{{escaped-json-character}}|5)* \\]\" )",
+			"$beginComment": "An expression must start with [ (no whitespace before), not start with [[, and end with ] (no whitespace after)",
 			"end": "\\]\"",
 			"beginCaptures": {
 				"0": {