Regular Expressions 101

Explanation
Pro

Non-Capturing Group(?:(?:25[0-5]|(?:2[0-4]|1[0-9]|[1-9]|)[0-9])(?:\.(?!$)|$)){4}

{4}repeats the group contents below exactly 4 times:

Non-Capturing Group(?:25[0-5]|(?:2[0-4]|1[0-9]|[1-9]|)[0-9])

1st Alternative25[0-5]

25the literal text 25

Character Class[0-5]

0-5one character from 0 (48₁₀) to 5 (53₁₀)

2nd Alternative(?:2[0-4]|1[0-9]|[1-9]|)[0-9]

Non-Capturing Group(?:2[0-4]|1[0-9]|[1-9]|)

1st Alternative2[0-4]

2the literal 2 (50₁₀ / 62₈ / 32₁₆)

Character Class[0-4]

0-4one character from 0 (48₁₀) to 4 (52₁₀)

2nd Alternative1[0-9]

1the literal 1 (49₁₀ / 61₈ / 31₁₆)

Character Class[0-9]

0-9one character from 0 (48₁₀) to 9 (57₁₀)

3rd Alternative[1-9]

Character Class[1-9]

1-9one character from 1 (49₁₀) to 9 (57₁₀)

4th Alternative — matches an empty string

Character Class[0-9]

0-9one character from 0 (48₁₀) to 9 (57₁₀)

Non-Capturing Group(?:\.(?!$)|$)

1st Alternative\.(?!$)

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

Negative Lookahead(?!$)

$end of line

2nd Alternative$

$end of line

Global pattern flags

g modifier: global. Finds all matches instead of stopping after the first

m modifier: multiline. Causes ^ and $ to match the start and end of each line, not only the start and end of the string

Ready to run

26 tokens

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