Backrefrence in regular expression(java)

Question

This regex is used to validate ip address

regex = "^([01]?\\d\\d?|2[0-4]\\d|25[0-5])\\.([01]?\\d\\d?|2[0-4]\\d|25[0-5])\\.([01]?\\d\\d?|2[0-4]\\d|25[0-5])\\.([01]?\\d\\d?|2[0-4]\\d|25[0-5])$"

/!\ modified version /!\

regex = "^([01]?\\d\\d?|2[0-4]\\d|25[0-5])\\.(\\1)\\.(\\1)\\.(\\1)$"

But the modified version is not working!! why it is not working??

Define "not working". Also \\1 expects exact same value as matched by group 1, so in case of regex like ([a-c])\\1 it can match only aa bb cc, not ac since group 1 would hold a in this case. — Pshemo
– Pshemo, Commented Jun 1, 2015 at 22:44
if i give input = 000.12.12.034 , the regex is returning false.. but it should return true. — syed_sami
– syed_sami, Commented Jun 1, 2015 at 22:49
I understand what you are trying to do, it's not possible with java regex since it has not the feature to reuse sub-patterns. But keep in mind that the assertion "a shorter pattern is faster" is totally false. — Casimir et Hippolyte
– Casimir et Hippolyte, Commented Jun 1, 2015 at 23:11

Pshemo · Accepted Answer · 2015-06-01 23:01:27Z

2

\\1 expects exact same value as matched by group 1, so your regex can find only IPs like

123.123.123.123

since first element is stored in group 1 and rest of elements needs to match it.

What you seem to be looking for is limited repetitions {n} like

String regex = "([01]?\\d\\d?|2[0-4]\\d|25[0-5])(\\.([01]?\\d\\d?|2[0-4]\\d|25[0-5])){3}";

BTW I skipped ^ and $ flags since matches(regex) adds them by default, and if you want to use matcher.find() then you probably don't need them since you are probably looking for IP inside some text.

edited Jun 1, 2015 at 23:01

answered Jun 1, 2015 at 22:47

Pshemo

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3 Comments

syed_sami Over a year ago

i tried your regex but its giving me an error can't find symbol ([01]?\\d\\d?|2[0-4]\\d|25[0-5])(\\.([01]?\\d\\d?|2[0-4]\\d|25[0-5])){3} and the up arrow pointing at the 25[0-5]

Pshemo Over a year ago

@syed_sami How exactly are you using this regex? Did you used it as String?

syed_sami Over a year ago

i'm using it as a string.. just like this String ip = (my input here); System.out.println(ip.matches(regex));

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