def srange(s):
    r"""
    Helper to easily define string ranges for use in Word construction.  Borrows
    syntax from regexp '[]' string range definitions::
        srange("[0-9]")   -> "0123456789"
        srange("[a-z]")   -> "abcdefghijklmnopqrstuvwxyz"
        srange("[a-z$_]") -> "abcdefghijklmnopqrstuvwxyz$_"
    The input string must be enclosed in []'s, and the returned string is the expanded
    character set joined into a single string.
    The values enclosed in the []'s may be:
     - a single character
     - an escaped character with a leading backslash (such as C{\-} or C{\]})
     - an escaped hex character with a leading C{'\x'} (C{\x21}, which is a C{'!'} character) 
         (C{\0x##} is also supported for backwards compatibility) 
     - an escaped octal character with a leading C{'\0'} (C{\041}, which is a C{'!'} character)
     - a range of any of the above, separated by a dash (C{'a-z'}, etc.)
     - any combination of the above (C{'aeiouy'}, C{'a-zA-Z0-9_$'}, etc.)
    """
    _expanded = lambda p: p if not isinstance(p,ParseResults) else ''.join(unichr(c) for c in range(ord(p[0]),ord(p[1])+1))
    try:
        return "".join(_expanded(part) for part in _reBracketExpr.parseString(s).body)
    except Exception:
        return ""

    Helper to easily define string ranges for use in Word construction.  Borrows
    syntax from regexp '[]' string range definitions::
        srange("[0-9]")   -> "0123456789"
        srange("[a-z]")   -> "abcdefghijklmnopqrstuvwxyz"
        srange("[a-z$_]") -> "abcdefghijklmnopqrstuvwxyz$_"
    The input string must be enclosed in []'s, and the returned string is the expanded
    character set joined into a single string.
    The values enclosed in the []'s may be:
     - a single character
     - an escaped character with a leading backslash (such as C{\-} or C{\]})
     - an escaped hex character with a leading C{'\x'} (C{\x21}, which is a C{'!'} character) 
         (C{\0x##} is also supported for backwards compatibility) 
     - an escaped octal character with a leading C{'\0'} (C{\041}, which is a C{'!'} character)
     - a range of any of the above, separated by a dash (C{'a-z'}, etc.)
     - any combination of the above (C{'aeiouy'}, C{'a-zA-Z0-9_$'}, etc.)
    
src/pkg_resources/_vendor/pyparsing.py