Java Regex to match String password

Question

I have recently encountered this question in the text book:

I am suppose to write a method to check if a string have:

at least ten characters
only letters and digits
at least three digits

I am trying to solve it by Regx, rather than iterating through every character; this is what I got so far:

String regx = "[a-z0-9]{10,}";

But this only matches the first two conditions. How should I go about the 3rd condition?

Possible duplicate of Password matching with regex

Ken White
– Ken White

2017-07-24 00:48:18 +00:00
Commented Jul 24, 2017 at 0:48 — Ken White
– Ken White, Commented Jul 24, 2017 at 0:48
Thank you so much, should have searched before asking ><

Milk_QD
– Milk_QD

2017-07-24 05:10:21 +00:00
Commented Jul 24, 2017 at 5:10 — Milk_QD
– Milk_QD, Commented Jul 24, 2017 at 5:10

Gerry · Accepted Answer · 2017-07-24 01:01:51Z

4

You could use a positive lookahead for 3rd condition, like this:

^(?=(?:.*\d){3,})[a-z0-9]{10,}$

^ indicates start of string.
(?= ... ) is the positive lookahead, which will search the whole string to match whatever is between (?= and ).
(?:.*\d){3,} matches at least 3 digits anywhere in the string.
- .*\d matches a digit preceded by any (or none) character (if omitted then only consecutive digits would match).
- {3,} matches three or more of .*\d.
- (?: ... ) is a non-capturing group.
$ indicates end of string.

edited Jul 24, 2017 at 1:01

answered Jul 24, 2017 at 0:42

Gerry

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4 Comments

Milk_QD Over a year ago

Thank you so much for your detailed explanation first. I am still a bit lost about the positive lookahead and ?: as this is my first time encountering them. My current understanding of them is the positive lookahead to allow (?:.*\d){3,} to search the whole string. and the ?: allows the .*\d{3} to match any three digits but not remembering them, so it does not contribute to the final {10,}

Milk_QD Over a year ago

And this form of Regx can only be used with class Pattern in Java? Not with String class methods such as .matches() and .replaceAll()

Gerry Over a year ago

@Anlinyang Yes, you got the lookahead right, and the ?: allows to use parenthesis to group an expression (e.g. .*\d) without capturing it (you need to group ithe expresssion in order to apply {3,} on it). You could use the same expression without the ?: and it will still work.

Gerry Over a year ago

@Anlinyang I'm not sure, i'm not Java savvy, but s far as i understand String class methods accept simple regex, so i guess you will need to create a pattern first and match the string agsinst it.

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