@staticmethod
def _get_numpy_expr(expr):
    """Extracts numpy operations from the overall string representation."""
    if expr[:5] == 'log10':
        return 5, 'log10'
    for idx, char in enumerate(expr):
        if not char.isalpha():
            if char.isnumeric():
                raise ValueError('invalid expression')
            return idx, expr[:idx]
