Question

I have a regex for IP address validation, but I need a regex for prefix validation, where the expected form is “IP Address/Prefix”.

The conditions are:

1. The prefix value should not be greater than 128

2. The prefix value should be divisible by 4.

Could anyone please help me to create a regex for prefix validation?

Answer 1

AFAIK you can’t do calculations using regex (e.g. number % 4 == 0 ?). Thus you’d have to use a pattern that gets all possible string combinations.

Try that one: \b[048]\b|\b[13579][26]\b|\b[2468][048]\b|\b1[02][048]\b|\b11[26]\b

• \b[048]\b matches 0, 4 and 8
• \b[13579][26]\b matches 12, 16, 32, 26 etc.
• \b[2468][048]\b matches 20, 24, 28, 40, 44, 48 etc.
• \b1[02][048]\b matches 100, 104, 108, 120, 124, 128
• \b11[26]\b matches 112 and 116

Note the \b which defines the whole word (in your case the prefix/suffix) must match the pattern. Without it, 136 might match [13579][26], for example.

Edit: to allow leading zeros change the pattern to: \b0{0,2}[048]\b|\b0?[13579][26]\b|\b0?[2468][048]\b|\b1[02][048]\b|\b11[26]\b (note that 0{0,2} could also be written as 0?0?).

Edit 2: you might get rid of the \b if you split the ip address and only have a string containing the prefix/suffix. If you then call matches(...) you should be fine without the \b.

Pattern for matches(...) calls (no \b, allows leading zeros):
0{0,2}[048]|0?[13579][26]|0?[2468][048]|1[02][048]|11[26]