added \xHH escape character to strings, allow strings of length zero.

compiler/src/prog8/ast/antlr/Antr2Kotlin.kt

@@ -647,7 +647,21 @@ private fun prog8Parser.VardeclContext.toAst(): VarDecl {
     )
 }
 
-internal fun escape(str: String) = str.replace("\t", "\\t").replace("\n", "\\n").replace("\r", "\\r")
+internal fun escape(str: String): String {
+    val es2 = str.replace("\t", "\\t").replace("\n", "\\n").replace("\r", "\\r")
+    val es = str.map {
+        when(it) {
+            '\t' -> "\\t"
+            '\n' -> "\\n"
+            '\r' -> "\\r"
+            '"' -> "\\\""
+            in '\u8000'..'\u80ff' -> "\\x" + (it.toInt() - 0x8000).toString(16).padStart(2, '0')
+            in '\u0000'..'\u00ff' -> it.toString()
+            else -> "\\u" + it.toInt().toString(16).padStart(4, '0')
+        }
+    }
+    return es.joinToString("")
+}
 
 internal fun unescape(str: String, position: Position): String {
     val result = mutableListOf<Char>()
@@ -664,7 +678,7 @@ internal fun unescape(str: String, position: Position): String {
                 'u' -> {
                     "${iter.nextChar()}${iter.nextChar()}${iter.nextChar()}${iter.nextChar()}".toInt(16).toChar()
                 }
-                '$' -> {
+                'x' -> {
                     // special hack 0x8000..0x80ff  will be outputted verbatim without encoding
                     val hex = ("" + iter.nextChar() + iter.nextChar()).toInt(16)
                     (0x8000 + hex).toChar()

compiler/src/prog8/ast/processing/AstIdentifiersChecker.kt

@@ -143,8 +143,8 @@ internal class AstIdentifiersChecker(private val program: Program, private val e
     }
 
     override fun visit(string: StringLiteralValue) {
-        if (string.value.length !in 1..255)
-            errors.err("string literal length must be between 1 and 255", string.position)
+        if (string.value.length > 255)
+            errors.err("string literal length max is 255", string.position)
 
         super.visit(string)
     }

docs/source/syntaxreference.rst

@@ -416,7 +416,7 @@ There are several escape sequences available to put special characters into your
 - ``\r`` - carriage return character (more or less the same as newline if printing to the screen)
 - ``\"`` - quote character (otherwise it would terminate the string)
 - ``\uHHHH`` - a unicode codepoint \u0000 - \uffff (16-bit hexadecimal)
-- ``\$HH`` - 8-bit hex value that will be copied verbatim *without encoding*
+- ``\xHH`` - 8-bit hex value that will be copied verbatim *without encoding*
 
 
 Operators