1  # Copyright 2004-2005 Elemental Security, Inc. All Rights Reserved.
       2  # Licensed to PSF under a Contributor Agreement.
       3  
       4  """Safely evaluate Python string literals without using eval()."""
       5  
       6  import re
       7  
       8  simple_escapes = {"a": "\a",
       9                    "b": "\b",
      10                    "f": "\f",
      11                    "n": "\n",
      12                    "r": "\r",
      13                    "t": "\t",
      14                    "v": "\v",
      15                    "'": "'",
      16                    '"': '"',
      17                    "\\": "\\"}
      18  
      19  def escape(m):
      20      all, tail = m.group(0, 1)
      21      assert all.startswith("\\")
      22      esc = simple_escapes.get(tail)
      23      if esc is not None:
      24          return esc
      25      if tail.startswith("x"):
      26          hexes = tail[1:]
      27          if len(hexes) < 2:
      28              raise ValueError("invalid hex string escape ('\\%s')" % tail)
      29          try:
      30              i = int(hexes, 16)
      31          except ValueError:
      32              raise ValueError("invalid hex string escape ('\\%s')" % tail) from None
      33      else:
      34          try:
      35              i = int(tail, 8)
      36          except ValueError:
      37              raise ValueError("invalid octal string escape ('\\%s')" % tail) from None
      38      return chr(i)
      39  
      40  def evalString(s):
      41      assert s.startswith("'") or s.startswith('"'), repr(s[:1])
      42      q = s[0]
      43      if s[:3] == q*3:
      44          q = q*3
      45      assert s.endswith(q), repr(s[-len(q):])
      46      assert len(s) >= 2*len(q)
      47      s = s[len(q):-len(q)]
      48      return re.sub(r"\\(\'|\"|\\|[abfnrtv]|x.{0,2}|[0-7]{1,3})", escape, s)
      49  
      50  def test():
      51      for i in range(256):
      52          c = chr(i)
      53          s = repr(c)
      54          e = evalString(s)
      55          if e != c:
      56              print(i, c, s, e)
      57  
      58  
      59  if __name__ == "__main__":
      60      test()