Python FEN chess Validation with regular expression

Python FEN chess Validation with regular expression

def fenPass(fen):
    """
    """
    regexMatch=re.match('\s*^(((?:[rnbqkpRNBQKP1-8]+\/){7})[rnbqkpRNBQKP1-8]+)\s([b|w])\s([K|Q|k|q]{1,4})\s(-|[a-h][1-8])\s(\d+\s\d+)$', fen)
    if  regexMatch:
        regexList = regexMatch.groups()
        fen = regexList[0].split("/")
        if len(fen) != 8:
            raise ValueError("expected 8 rows in position part of fen: {0}".format(repr(fen)))

        for fenPart in fen:
            field_sum = 0
            previous_was_digit, previous_was_piece = False,False

            for c in fenPart:
                if c in ["1", "2", "3", "4", "5", "6", "7", "8"]:
                    if previous_was_digit:
                        raise ValueError("two subsequent digits in position part of fen: {0}".format(repr(fen)))
                    field_sum += int(c)
                    previous_was_digit = True
                    previous_was_piece = False
                elif c == "~":
                    if not previous_was_piece:
                        raise ValueError("~ not after piece in position part of fen: {0}".format(repr(fen)))
                    previous_was_digit, previous_was_piece = False,False
                elif c.lower() in ["p", "n", "b", "r", "q", "k"]:
                    field_sum += 1
                    previous_was_digit = False
                    previous_was_piece = True
                else:
                    raise ValueError("invalid character in position part of fen: {0}".format(repr(fen)))

            if field_sum != 8:
                raise ValueError("expected 8 columns per row in position part of fen: {0}".format(repr(fen)))  

    else: raise ValueError("fen doesn`t match follow this example: rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1 ")  


def main():
    try:
        fenPass('rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1')
    except ValueError as e:
        print e.message
        
if __name__ == '__main__':
    main()

@PanCave You can remove the condition since it is redundant. The regex expression matching will never allow ~ in the FEN notation

For regexList = regexMatch.groups(), it may be better to replace this with regexList = fen.split().

For this FEN: "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1", regexMatch.groups() sets regexList equal to:

["rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR", "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/", "w", "KQkq", "-", "0 1"]

In the gist provided this doesn't make any difference, but if someone wanted to test more than the first portion of their FEN, the subsequent elements might not be what they expect. In terms of FEN notation, the second element shouldn't exist, and the "0 1" should be two different elements.