Extracing IP addresses as whole words with POSIX BRE/ERE regex

Question

I am trying to match IP addresses found in the output of traceroute by means of a regex. I'm not trying to validate them because it's safe enough to assume traceroute is valid (i.e. is not outputting something like 999.999.999.999. I'm trying the following regex:

([0-9]{1,3}.?){4}

I'm testing it in regex101 and it does validate an IP address. However, when I try

echo '192.168.1.1 foobar' | grep '([0-9]{1,3}.?){4}' 

I get nothing. What am I missing?

Answer 1

You used a POSIX ERE pattern, but did not pass -E option to have grep use the POSIX ERE flavor. Thus, grep used POSIX BRE instead, where you need to escape {n,m} quantifier and (...) to make them be parsed as special regex operators.

Note you need to escape a . so that it could only match a literal dot.

To make your pattern work with grep the way you wanted you could use:

grep -E '([0-9]{1,3}\.?){4}'      # POSIX ERE
grep '\([0-9]\{1,3\}\.\?\)\{4\}'  # POSIX BRE version of the same regex

See an online demo.

However, this regex will also match a string of several digits because the . is optional.

You may solve it by unrolling the pattern as

grep -E '[0-9]{1,3}(\.[0-9]{1,3}){3}'      # POSIX ERE
grep '[0-9]\{1,3\}\(\.[0-9]\{1,3\}\)\{3\}' # POSIX BRE

See another demo.

Basically, it matches:

• [0-9]{1,3} - 1 to 3 occurrences of any ASCII digit
• (\.[0-9]{1,3}){3} - 3 occurrences of:
  • \. - a literal .
  • [0-9]{1,3} - 1 to 3 occurrences of any ASCII digit

To make sure you only match valid IPs, you might want to use a more precise IP matching regex:

grep -E '\b(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)(\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)){3}\b' # POSIX ERE

See this online demo.

You may further tweak it with word boundaries (can be \< / \> or \b), etc.

To extract the IPs use -o option with grep: grep -oE 'ERE_pattern' file / grep -o 'BRE_pattern' file.

Answer 2

To make a more effective validation, it is better to use a function instead of a simple regex match:

#!/bin/bash
is_valid_ip() {
  local arr element
  IFS=. read -r -a arr <<< "$1"                  # convert ip string to array
  [[ ${#arr[@]} != 4 ]] && return 1              # doesn't have four parts
  for element in "${arr[@]}"; do
    [[ $element =~ ^[0-9]+$ ]]       || return 1 # non numeric characters found
    [[ $element =~ ^0[1-9]+$ ]]      || return 1 # 0 not allowed in leading position if followed by other digits, to prevent it from being interpreted as on octal number
    ((element < 0 || element > 255)) && return 1 # number out of range
  done
  return 0
}

You can invoke this as:

while read -r ip; do
  is_valid_ip "$ip" && printf '%s\n' "$ip" 
done < <(your command that extracts ip address like strings)

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Related:

• Check for IP validity