UNIX for Advanced & Expert Users

What do you think of this regex to match IP address? I have been reading up on regex and have seen some really long ones for IP. Would this fail in any scenarios?

([0-9]+\.){3}[0-9]*

Yeah,it has some simple mistake.You need to change the * part in your regexp to + as follows.

Code:

([0-9]+\.){3}[0-9]+

But,it will match all the range of IP addresses even though your network ID contains 1 digit or two digit or three digits ,et.c.,If you want the specified number of digits for your network id,then use the number instead of + in your regexp as follows.

Code:

([0-9]{3}\.){3}[0-9]+

The above code will match the IP addresses which having 3 digits in the network address.For ex,

Code:

192.168.128.9 
192.168.128.10
192.168.128.100
 ...

Code:

([0-9]{1,3}\.){3}[0-9]+

since you can give the ip address as

Code:

1.2.3.4
1.2.2.46
192.168.1.1

That means the ip can be denoted by 1 to 3 numbers in each division,
I think this must be the perfect regex for IP addresses.

Following line also match the IP address.
It will match any IP address.

[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}

See the following sample PERL code. the regular expression will match the ip address.

Code:

 perl -e 'foreach(<>){ print if /^[0-9]{1,3}[.][0-9]{1,3}[.][0-9]{1,3}[.][0-9]{1,3}$/ }'

[OR]

Code:

 perl -e 'foreach(<>){ print if /^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$/ }'

Use this regular expression in egrep

Code:

egrep "^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$"

It is important to note -
the perl regex engine is not identical to the engine used for grep.

There are a lot of flavors of regex out there.

You may want to check out on google or find a copy of J Freidl's 'Mastering Regular Expressions':

nondeterministic finite-state automata NFA
deterministic finite-state automata DFA