Regular Expressions 101

Explanation

Pro

^start of line

Negative Lookahead(?![10]\d{3})

Character Class[10]

1the literal 1 (49₁₀ / 61₈ / 31₁₆)

0the literal 0 (48₁₀ / 60₈ / 30₁₆)

\d{3}a digit, repeated exactly 3 times

Negative Lookahead(?!2(?=01[0-6]))

2the literal 2 (50₁₀ / 62₈ / 32₁₆)

Positive Lookahead(?=01[0-6])

01the literal text 01

Character Class[0-6]

0-6one character from 0 (48₁₀) to 6 (54₁₀)

Negative Lookahead(?!2(?=00\d))

2the literal 2 (50₁₀ / 62₈ / 32₁₆)

Positive Lookahead(?=00\d)

00the literal text 00

\da digit

\d{4}a digit, repeated exactly 4 times

$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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25 tokens

^(?![10]\d{3})(?!2(?=01[0-6]))(?!2(?=00\d))\d{4}$

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