Regular Expressions 101

Explanation
Pro

Negative Lookahead(?!^(\d{1,3}\.){3}\d{1,3}$)

^start of string

Group 1(\d{1,3}\.){3}

{3}repeats the group contents below exactly 3 times:

\d{1,3}a digit, repeated between 1 and 3 times

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

\d{1,3}a digit, repeated between 1 and 3 times

$end of string

Group 2(^[a-z0-9]([a-z0-9-]*(\.[a-z0-9])?)*$)

^start of string

Character Class[a-z0-9]

a-zone character from a (97₁₀) to z (122₁₀) (case sensitive)

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

Group 3([a-z0-9-]*(\.[a-z0-9])?)*

*repeats the group contents below any number of times:

Character Class[a-z0-9-]*

*matches any number of characters from the set below:

a-zone character from a (97₁₀) to z (122₁₀) (case sensitive)

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

-the literal - (45₁₀ / 55₈ / 2D₁₆)

Group 4(\.[a-z0-9])?

?repeats the group contents below at most once:

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

Character Class[a-z0-9]

a-zone character from a (97₁₀) to z (122₁₀) (case sensitive)

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

$end of string

Global pattern flags

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

Ready to run

26 tokens

(?!^(\d{1,3}\.){3}\d{1,3}$)(^[a-z0-9]([a-z0-9-]*(\.[a-z0-9])?)*$)