Regular Expressions 101

^start of line

Group 1((2(?!5?[6-9])|1|(?!0\B))\d\d?\.?\b){4}

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

Group 2(2(?!5?[6-9])|1|(?!0\B))

1st Alternative2(?!5?[6-9])

2the literal 2 (50₁₀ / 62₈ / 32₁₆)

Negative Lookahead(?!5?[6-9])

5?optionally matches the literal 5 (53₁₀ / 65₈ / 35₁₆)

Character Class[6-9]

6-9one character from 6 (54₁₀) to 9 (57₁₀)

2nd Alternative1

1the literal 1 (49₁₀ / 61₈ / 31₁₆)

3rd Alternative(?!0\B)

Negative Lookahead(?!0\B)

0the literal 0 (48₁₀ / 60₈ / 30₁₆)

\Bnot an ASCII word boundary

\da digit

\d?optionally matches a digit

\.?optionally matches the literal . (46₁₀ / 56₈ / 2E₁₆)

\ban ASCII word boundary

$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

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26 tokens

Regular Expression

^((2(?!5?[6-9])|1|(?!0\B))\d\d?\.?\b){4}$

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