str1 = 'a23'
if str1[0].isdecimal():
    print('Yes')
else:
    print('No')

class Error(Exception):
    """Base class for other exceptions"""
    pass

class lenghtError(Error):
    """incorrect length"""
    pass
def testPhone(te):
    for i in range(11):
        if i == 4 or i == 8:
            if str(te[i])=='-':
                a = True
            else:
                return False
        if i != 4 or i != 8:
            if str(te[i]).isdecimal() == True:
                a = True
            else:
                return False
    return a
            
        
        

while True:
    try:
        tele = str(input("Enter a number: "))
        if len(tele) != 12:
            raise lenghtError
            print('length error')
        if testPhone(tele)==True:
            break
        else:
            print('format error')
        
        
    except Error as e:
        print(e)
    else:
        break

print("format correct")

? = can be ignored

‘*’ = non to many

‘+’=1 to many

{N}=()times N number , must be satisfied

search is to get the first one only

findall is to get all