Regular Expressions 101

Explanation
Pro

Positive Lookbehind(?<=\D|^)

1st Alternative\D

\Dany character that's not a digit

2nd Alternative^

^start of line

Group year(?<year>\d{4})

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

Group sep(?<sep>[^\w\s])

Negated Character Class[^\w\s]

\wany word character

\sany whitespace character

Group month(?<month>1[0-2]|0[1-9])

1st Alternative1[0-2]

1the literal 1 (49₁₀ / 61₈ / 31₁₆)

Character Class[0-2]

0-2one character from 0 (48₁₀) to 2 (50₁₀)

2nd Alternative0[1-9]

0the literal 0 (48₁₀ / 60₈ / 30₁₆)

Character Class[1-9]

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

\k<sep>the text saved by group sep

1st Alternative0[1-9]

0the literal 0 (48₁₀ / 60₈ / 30₁₆)

Character Class[1-9]

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

2nd Alternative[12][0-9]

Character Class[12]

1the literal 1 (49₁₀ / 61₈ / 31₁₆)

2the literal 2 (50₁₀ / 62₈ / 32₁₆)

Character Class[0-9]

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

3rd Alternative(?<=11\k<sep>|[^1][4-9]\k<sep>)30

Positive Lookbehind(?<=11\k<sep>|[^1][4-9]\k<sep>)

1st Alternative11\k<sep>

11the literal text 11

\k<sep>the text saved by group sep

2nd Alternative[^1][4-9]\k<sep>

Negated Character Class[^1]

1the literal 1 (49₁₀ / 61₈ / 31₁₆)

Character Class[4-9]

4-9one character from 4 (52₁₀) to 9 (57₁₀)

\k<sep>the text saved by group sep

30the literal text 30

4th Alternative(?<=1[02]\k<sep>|[^1][13578]\k<sep>)3[01]

Positive Lookbehind(?<=1[02]\k<sep>|[^1][13578]\k<sep>)

1st Alternative1[02]\k<sep>

2nd Alternative[^1][13578]\k<sep>

3the literal 3 (51₁₀ / 63₈ / 33₁₆)

Character Class[01]

Positive Lookahead(?=\D|$)

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

Unit Test Summary

Ready to run

Quick Reference

26 tokens

Regular Expression

(?<=\D|^)(?<year>\d{4})(?<sep>[^\w\s])(?<month>1[0-2]|0[1-9])\k<sep>(?<day>0[1-9]|[12][0-9]|(?<=11\k<sep>|[^1][4-9]\k<sep>)30|(?<=1[02]\k<sep>|[^1][13578]\k<sep>)3[01])(?=\D|$)

Regular Expression

Unit Tests