regex101: Ascending and Descending order

Pro

Positive Lookahead(?=\b\d{8}\b)

\ba Unicode word boundary

\d{8}a digit, repeated exactly 8 times

\ba Unicode word boundary

Non-Capturing Group(?:(?:9?8?7?6?5?4?3?2?1?0?)|(?:0?1?2?3?4?5?6?7?8?9?))

1st Alternative(?:9?8?7?6?5?4?3?2?1?0?)

Non-Capturing Group(?:9?8?7?6?5?4?3?2?1?0?)

9?optionally matches the literal 9 (57₁₀ / 71₈ / 39₁₆)

8?optionally matches the literal 8 (56₁₀ / 70₈ / 38₁₆)

7?optionally matches the literal 7 (55₁₀ / 67₈ / 37₁₆)

6?optionally matches the literal 6 (54₁₀ / 66₈ / 36₁₆)

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

4?optionally matches the literal 4 (52₁₀ / 64₈ / 34₁₆)

3?optionally matches the literal 3 (51₁₀ / 63₈ / 33₁₆)

2?optionally matches the literal 2 (50₁₀ / 62₈ / 32₁₆)

1?optionally matches the literal 1 (49₁₀ / 61₈ / 31₁₆)

0?optionally matches the literal 0 (48₁₀ / 60₈ / 30₁₆)

2nd Alternative(?:0?1?2?3?4?5?6?7?8?9?)

Non-Capturing Group(?:0?1?2?3?4?5?6?7?8?9?)

0?optionally matches the literal 0 (48₁₀ / 60₈ / 30₁₆)

1?optionally matches the literal 1 (49₁₀ / 61₈ / 31₁₆)

2?optionally matches the literal 2 (50₁₀ / 62₈ / 32₁₆)

3?optionally matches the literal 3 (51₁₀ / 63₈ / 33₁₆)

4?optionally matches the literal 4 (52₁₀ / 64₈ / 34₁₆)

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

6?optionally matches the literal 6 (54₁₀ / 66₈ / 36₁₆)

7?optionally matches the literal 7 (55₁₀ / 67₈ / 37₁₆)

8?optionally matches the literal 8 (56₁₀ / 70₈ / 38₁₆)

9?optionally matches the literal 9 (57₁₀ / 71₈ / 39₁₆)

Positive Lookbehind(?<=\d{8})

\d{8}a digit, repeated exactly 8 times

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

27 tokens

Regular Expression

(?=\b\d{8}\b)(?:(?:9?8?7?6?5?4?3?2?1?0?)|(?:0?1?2?3?4?5?6?7?8?9?))(?<=\d{8})

New

This regex is to match Ascending or descending order of 8 digit number
01234567   0 to 7
12345678   1 to 8
23456789   2 to 9
12456789   missing 3
01236789   missing 4,5
23456789   missing 0,1
1234567
12345
012345678
123456789
0123456789
98765432   9 to 2
87654321   8 to 1
76543210   7 to 0
98765321   missing 4
96543210   missing 7,8
98765432   missing 0,1