File input and Data reading , help please!!

[youichi]

I have a file with the following content
123456789,123
234567891,345
345678901,234
124356877,456

I have to read these data and check the validity
the following conditions must be satisfied:

1. there is one and only one comma
2. all symbols except comma are digits(0-9)
3. there are exactly 13 digits before comma

I cannot figure out the method, please help!! thank you