Using grep to get ip address

Question

I am learning regular expressions and as aa practice I tried grep'ing an ip address but it fails to get any results. I tried it out on VSCode and it works. Anyone can enligheten me where did I go wrong?

echo 192.168.1.1 | grep -e '\d+\.\d+\.\d+\.\d+'

Related: [Why does my regular expression work in X but not in Y?](https://unix.stackexchange.com/questions/119905/why-does-my-regular-expression-work-in-x-but-not-in-y) - in particular note that the `+` quantifier is part of the *extended* regular expression syntax and `\d` is from the perl (PCRE) extension set — steeldriver, Jan 10 '21 at 18:29
Note that when you figure out how to get your `grep` to understand `\d` as "a digit", you still would match e.g. `000.0002222.020202020.020.0.0.0.0.0` with that expression. — Kusalananda, Jan 10 '21 at 18:53
Thanks for answering, didn't know there were so many types. this is really confusing. — ajmandourah, Jan 11 '21 at 09:35

score 1 · Accepted Answer · answered Jan 10 '21 at 18:38

1

Use grep -P:

-P, --perl-regexp         PATTERNS are Perl regular expressions

answered Jan 10 '21 at 18:38

GAD3R

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It works. Thanks – ajmandourah Jan 11 '21 at 09:35

Eduardo Trápani · Answer 2 · 2021-01-10T19:52:03.490

Note that regular expressions are not really meant for parsing things like IPv4 addresses. You can try to do it, to a certain level of accuracy:

Your expression \d+\.\d+\.\d+\.\d+ would match any four groups of digits separated by a dot, if \d matched a digit on its own.

Keep in mind that regular expressions have many flavors and results may vary, depending on the engine being used. When you use grep -P as suggested in another answer, you change the parsing engine.

In Matching IPv4 Addresses - Regular Expressions Cookbook by O'Reilly you have some examples.

This checks for (just) an IP address, not many checks though, 299.299.111.1 would pass:

^(?:[0-9]{1,3}\.){3}[0-9]{1,3}$

The other examples in the page try to narrow down the detection to get a valid IP address, see what they propose for accurate extraction from longer text and you'll know why IPv4 addresses are not a good playground for regular expressions:

\b(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}↵
(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\b

Using grep to get ip address

2 Answers2