Regex for alphanumeric string including at least one digit - regex

Question

I'm looking for a regex with at the conditions:
a) minimum 13 alphanumeric characters
b) maximum 17 alphanumeric
characters
c) and at least 1 digit.
This regex fulfils a) and b). How can it fulfil also condition c)?
^[a-zA-Z0-9]{13,17}$
Example input texts:
# matching
123456789abcd
123456789abcdef
123456789abcdefg
# no match: too long
123456789abcdefgef
# no match: no digit
abcdefghijklmno
# no match: not alphanumeric only
123456789#abcdefg
The flavor is Java 8.

Answer 1

It sounds like you are trying to make a password checker. I suggest that you NOT try to do it all in one single regex.
Check your input against two different regexes that must both match:
^[a-zA-Z0-9]{13,17}$ # 13-17 alphanumerics
and
[0-9] # at least one digit
Compared to this suggestion from another answer...
^(?=[a-zA-Z]*[0-9])(?=[0-9]*[a-zA-Z])[[:alnum:]]{13,17}$
... it's so much clearer that way, and easier to change when your rules change in the future.

Answer 2

Regex:
^(?=[a-zA-Z]*[0-9])(?=[0-9]*[a-zA-Z])[[:alnum:]]{13,17}$
Explanation:
^(?=[a-zA-Z]*[0-9]) - Lookahead for zero or more letters followed by a number, somewhere in the string, note this is faster than the (?=.*[0-9]) suggested in another answer.
(?=[0-9]*[a-zA-Z]) - as above but looking for at least one letter (I assume you want this, although it wasn't specified in your answer.
[[:alnum:]]{13,17}$ - The actual match bit; this is just your regex ([a-zA-Z0-9]{13,17}$) in short-hand.
If it's more likely that a string will not have a letter then swapping the order of the lookaheads should make it faster. The current order would be best if a missing number is the most likely fail condition (as the regex will fail faster).
See https://www.rexegg.com/regex-lookarounds.html#password for a detailed explanation.
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