def countedArray( expr, intExpr=None ):
    """
    Helper to define a counted list of expressions.
    This helper defines a pattern of the form::
        integer expr expr expr...
    where the leading integer tells how many expr expressions follow.
    The matched tokens returns the array of expr tokens as a list - the leading count token is suppressed.
    
    If C{intExpr} is specified, it should be a pyparsing expression that produces an integer value.

    Example::
        countedArray(Word(alphas)).parseString('2 ab cd ef')  # -> ['ab', 'cd']

        # in this parser, the leading integer value is given in binary,
        # '10' indicating that 2 values are in the array
        binaryConstant = Word('01').setParseAction(lambda t: int(t[0], 2))
        countedArray(Word(alphas), intExpr=binaryConstant).parseString('10 ab cd ef')  # -> ['ab', 'cd']
    """
    arrayExpr = Forward()
    def countFieldParseAction(s,l,t):
        n = t[0]
        arrayExpr << (n and Group(And([expr]*n)) or Group(empty))
        return []
    if intExpr is None:
        intExpr = Word(nums).setParseAction(lambda t:int(t[0]))
    else:
        intExpr = intExpr.copy()
    intExpr.setName("arrayLen")
    intExpr.addParseAction(countFieldParseAction, callDuringTry=True)
    return ( intExpr + arrayExpr ).setName('(len) ' + _ustr(expr) + '...')

    Helper to define a counted list of expressions.
    This helper defines a pattern of the form::
        integer expr expr expr...
    where the leading integer tells how many expr expressions follow.
    The matched tokens returns the array of expr tokens as a list - the leading count token is suppressed.
    
    If C{intExpr} is specified, it should be a pyparsing expression that produces an integer value.

    Example::
        countedArray(Word(alphas)).parseString('2 ab cd ef')  # -> ['ab', 'cd']

        # in this parser, the leading integer value is given in binary,
        # '10' indicating that 2 values are in the array
        binaryConstant = Word('01').setParseAction(lambda t: int(t[0], 2))
        countedArray(Word(alphas), intExpr=binaryConstant).parseString('10 ab cd ef')  # -> ['ab', 'cd']
    
src/pkg_resources/_vendor/pyparsing.py