Random Character Generation following a pattern

In this code I'm trying to get Python to generate a code that is either 4 numbers followed by 3 letter or 3 numbers followed by 3 letters.

def plate():
    char=[a-z0-9]
    while True:
    compile('(^|\s[0-9]{3-4}+-[a-z]{3}(\s|$)')
    yield char
    print(char)

plate()

When I run this program the shell displays.

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>>> 
>>>