IPv4 Address Regex Walkthrough

There are applications that gets input from the user prompting for an IP address such as 192.168.1.255. However, if the user inputs an incorrectly formatted IP address, then the application can crash if it tries to connect to a boggus address. Using this regex, it will verify if the inputted IP address is formatted correctly.

Summary

This is the regex:

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

An IP address such as 192.168.1.34 has four groups of numbers ranging from 0 to 999. Each of them are separated with a dot .. We will go over groups, quantifiers, and character ranges that are used for this regex along with smaller details such as the character escape.

Regex Components

Beginning of String Anchor

The ^ (caret) character goes at the beginning of the expression indicating the a match should occur only at the beginning of the string. It may also indicate a match at the beginning of a line of a multi-line string, yet this only works if the m (multiline) flag is present.

Non Captured Group

Anything in parenthesis () is a group of characters (substring) to match to. This allows you to quantify them or reference them later. However, the ?: symbol at the start of the group indicates that the group will not be referenced when performing post operations with regex such as when replacing, and this is why it’s called non capturing.

Character Set

Anything in square brackets [] defines a list of characters that it can be matched to. For example, a set like [wasd] will match a character equal to w, a, s, or d. A misconception of the character set is that, for example, a set like [wasd] will match a substring equal to wasd, yet that is not true because it can only match one character at a time.

Character range

Useful inside character sets is the character range specifier. As it implies, you can define a range of characters to match to, so you can, for example, match any lower case letter by writing [a-z]. The order of those characters follows the ASCII table.

Quantifier

Curly braces ({}) sets a rule to the previous character or substring on the quantity of it to be expected. A quantifier like {3} enforeces a count of characters before it to 3, and specifying a range is allowed by writing it as {<minimum>-<maximum>}. On our example, the quantifier {1-3} ensures a number to have a minimum of 1 digit to a maximum of 3 digits.

Escaped Character

When writing a regex, there are some reserved characters that define rules and patterns such as (), but you need to write that character literally so that it can be used to match in a string. The blackslash \ escapes that character so that the interperter does not read it as a symbol but rather a literal char. In this case, \. escapes the . character that is normally used as a wildcard symbol, but now it is read as just a dot (.).

End of String Anchor

The dollar sign $ rules that the match’s end should also be the string’s or line’s end. If there are characters or whitespace after the match (besides new line), it will disqualify it because it ends with it proceeding it.