Quick Start
Tutorial
Tools & Languages
Examples
Reference
Book Reviews
Examples
Regular Expressions Examples
Numeric Ranges
Floating Point Numbers
Email Addresses
IP Addresses
Valid Dates
Numeric Dates to Text
Credit Card Numbers
Matching Complete Lines
Deleting Duplicate Lines
Programming
Two Near Words
Pitfalls
Catastrophic Backtracking
Too Many Repetitions
Denial of Service
Making Everything Optional
Repeated Capturing Group
Mixing Unicode & 8-bit
More on This Site
Introduction
Regular Expressions Quick Start
Regular Expressions Tutorial
Replacement Strings Tutorial
Applications and Languages
Regular Expressions Examples
Regular Expressions Reference
Replacement Strings Reference
Book Reviews
Printable PDF
About This Site
RSS Feed & Blog
RegexMagic—Generate regular expressions matching ip addresses and ranges

How to Find or Validate an IP Address

Matching an IP address is another good example of a trade-off between regex complexity and exactness. \b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b will match any IP address just fine. But will also match 999.999.999.999 as if it were a valid IP address. If your regex flavor supports Unicode, it may even match ١٢٣.१२३.೧೨೩.๑๒๓. Whether this is a problem depends on the files or data you intend to apply the regex to.

Restricting and Capturing The Four IP Address Numbers

To restrict all 4 numbers in the IP address to 0..255, you can use the following regex. It stores each of the 4 numbers of the IP address into a capturing group. You can use these groups to further process the IP number. Free-spacing mode allows this to fit the width of the page.

\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]?)\.
  
(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]?)\b

The above regex allows one leading zero for numbers 10 to 99 and up to two leading zeros for numbers 0 to 9. Strictly speaking, IP addresses with leading zeros imply octal notation. So you may want to disallow leading zeros. This requires a slightly longer regex:

\b(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])\.
  
(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])\.
  
(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])\.
  
(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])\b

Restricting The Four IP Address Numbers Without Capturing Them

If you don’t need access to the individual numbers, you can shorten above 3 regexes with a quantifier to:

\b(?:\d{1,3}\.){3}\d{1,3}\b

\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

\b(?:(?:25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])\.){3}
  
(?:25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])\b

Checking User Input

The above regexes use word boundaries to make sure the first and last number in the IP address aren’t part of a longer sequence of alphanumeric characters. These regexes are appropriate for finding IP addresses in longer strings.

If you want to validate user input by making sure a string consists of nothing but an IP address then you need to replace the word boundaries with start-of-string and end-of-string anchors. You can use the dedicated anchors \A and \z if your regex flavor supports them:

\A(?:(?: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]?)\z

If not, you’ll have to use ^ and $ and make sure that the option for them to match at line breaks is off:

^(?:(?: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]?)$