Added strutils.unescape and fixed issue with strutils.escape.

lib/pure/strutils.nim

@@ -850,7 +850,7 @@ proc escape*(s: string, prefix = "\"", suffix = "\""): string {.noSideEffect,
   for c in items(s):
     case c
     of '\0'..'\31', '\128'..'\255':
-      add(result, '\\')
+      add(result, "\\x")
       add(result, toHex(ord(c), 2))
     of '\\': add(result, "\\\\")
     of '\'': add(result, "\\'")
@@ -858,6 +858,44 @@ proc escape*(s: string, prefix = "\"", suffix = "\""): string {.noSideEffect,
     else: add(result, c)
   add(result, suffix)
 
+proc unescape*(s: string, prefix = "\"", suffix = "\""): string {.noSideEffect,
+  rtl, extern: "nsuUnescape".} =
+  ## Unescapes a string `s`. This complements ``escape`` as it performs the
+  ## opposite operations.
+  ##
+  ## If `s` does not begin with ``prefix`` and end with ``suffix`` a EInvalidValue
+  ## exception will be raised.
+  result = newStringOfCap(s.len)
+  var i = 0
+  if s[0 .. prefix.len-1] != prefix:
+    raise newException(EInvalidValue,
+                       "String does not start with a prefix of: " & prefix)
+  i.inc()
+  while True:
+    if i == s.len-suffix.len: break
+    case s[i]
+    of '\\':
+      case s[i+1]:
+      of 'x':
+        let j = parseHexInt(s[i+2 .. i+3])
+        result.add(chr(j))
+        inc(i, 2)
+      of '\\':
+        result.add('\\')
+      of '\'':
+        result.add('\'')
+      of '\"':
+        result.add('\"')
+      else: result.add("\\" & s[i+1])
+      inc(i)
+    of '\0': break
+    else:
+      result.add(s[i])
+    i.inc()
+  if s[i .. -1] != suffix:
+    raise newException(EInvalidValue,
+                       "String does not end with a suffix of: " & suffix)
+
 proc validIdentifier*(s: string): bool {.noSideEffect,
   rtl, extern: "nsuValidIdentifier".} =
   ## returns true if `s` is a valid identifier. A valid identifier starts