regex101: NumberOnlyDate

Negative Lookbehind(?<!\d)

\da Unicode decimal digit

Non-Capturing Group(?: [0-9]{4} (?: (?:0[1-9]|1[0-2]) (?:0[1-9]|1[0-9]|2[0-8]) |(?:0[13-9]|1[0-2]) (?:29|30) |(?:0[13578]|1[02]) (?:31) ) | (?: [0-9]{2} (?: 0[48] |[2468][048] |[13579][26] ) |(?: 0[48] |[2468][048] |[13579][26] )00 ) 02 (?:29) )

1st Alternative [0-9]{4} (?: (?:0[1-9]|1[0-2]) (?:0[1-9]|1[0-9]|2[0-8]) |(?:0[13-9]|1[0-2]) (?:29|30) |(?:0[13578]|1[02]) (?:31) )

Character Class[0-9]{4}

{4}matches exactly 4 characters from the set below:

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

Non-Capturing Group(?: (?:0[1-9]|1[0-2]) (?:0[1-9]|1[0-9]|2[0-8]) |(?:0[13-9]|1[0-2]) (?:29|30) |(?:0[13578]|1[02]) (?:31) )

1st Alternative (?:0[1-9]|1[0-2]) (?:0[1-9]|1[0-9]|2[0-8])

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

1st Alternative0[1-9]

0the literal 0 (48₁₀ / 60₈ / 30₁₆)

Character Class[1-9]

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

2nd Alternative1[0-2]

1the literal 1 (49₁₀ / 61₈ / 31₁₆)

Character Class[0-2]

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

Non-Capturing Group(?:0[1-9]|1[0-9]|2[0-8])

1st Alternative0[1-9]

0the literal 0 (48₁₀ / 60₈ / 30₁₆)

Character Class[1-9]

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

2nd Alternative1[0-9]

1the literal 1 (49₁₀ / 61₈ / 31₁₆)

Character Class[0-9]

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

3rd Alternative2[0-8]

2the literal 2 (50₁₀ / 62₈ / 32₁₆)

Character Class[0-8]

0-8one character from 0 (48₁₀) to 8 (56₁₀)

2nd Alternative(?:0[13-9]|1[0-2]) (?:29|30)

Non-Capturing Group(?:0[13-9]|1[0-2])

1st Alternative0[13-9]

0the literal 0 (48₁₀ / 60₈ / 30₁₆)

Character Class[13-9]

1the literal 1 (49₁₀ / 61₈ / 31₁₆)

3-9one character from 3 (51₁₀) to 9 (57₁₀)

2nd Alternative1[0-2]

1the literal 1 (49₁₀ / 61₈ / 31₁₆)

Character Class[0-2]

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

Non-Capturing Group(?:29|30)

1st Alternative29

2nd Alternative30

3rd Alternative(?:0[13578]|1[02]) (?:31)

2nd Alternative (?: [0-9]{2} (?: 0[48] |[2468][048] |[13579][26] ) |(?: 0[48] |[2468][048] |[13579][26] )00 ) 02 (?:29)

Negative Lookahead(?!\d)

Global pattern flags

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

x modifier: extended. Ignores unescaped whitespace and comments outside character classes

Search reference

Regular Expression

(?<!\d)
    (?:
     [0-9]{4}
        (?:
            (?:0[1-9]|1[0-2])   (?:0[1-9]|1[0-9]|2[0-8])
           |(?:0[13-9]|1[0-2])  (?:29|30)
           |(?:0[13578]|1[02])  (?:31)
        )
    |
        (?:
            [0-9]{2}
            (?:
                0[48]
                |[2468][048]
                |[13579][26]
            )
           |(?:
               0[48]
              |[2468][048]
              |[13579][26]
            )00
        )
        02  (?:29)
    )
(?!\d)

New

aaa_20150229_bbb
aaa_2012-02-29_bbb
aaa_2000-02-29_bbb
aaa_20000229_bbb
20000228
2000-2-28
19120229
19000229


1696-2-29
16960229

20190228

aaa_1904229_bbb
aaa_18000229_bbb
aaa_17000229_bbb
aaa_16000229_bbb

2019-3-30
19960229
2019-02-28
20190228
20190229


aaa_20150130_bbb
aaa_20150131_bbb
aaa_2015-04-30_bbb
aaa_20150430_bbb
20190228
20190229
20200229
20000229
2000-02-29
2000-02-28

201911

Regular Expressions 101

Regular Expression

Extraction