Regular Expressions 101

Pro

^start of line

Positive Lookahead(?=(?:\D*\d){8}$)

Non-Capturing Group(?:\D*\d){8}

{8}repeats the group contents below exactly 8 times:

\D*any character that's not a digit, repeated any number of times

\da digit

$end of line

\d+a digit, repeated at least once

Non-Capturing Group(?:(?:-\d+){0,2}|(?:\s+\d+){0,1})

1st Alternative(?:-\d+){0,2}

Non-Capturing Group(?:-\d+){0,2}

{0,2}repeats the group contents below between 0 and 2 times:

-the literal - (45₁₀ / 55₈ / 2D₁₆)

\d+a digit, repeated at least once

2nd Alternative(?:\s+\d+){0,1}

Non-Capturing Group(?:\s+\d+){0,1}

{0,1}repeats the group contents below at most once:

\s+any whitespace character, repeated at least once

\d+a digit, repeated at least once

$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

Detailed match information will be displayed here automatically.

26 tokens

^(?=(?:\D*\d){8}$)\d+(?:(?:-\d+){0,2}|(?:\s+\d+){0,1})$

New

12345678
1234 5678
1234-5678
123-45-678
12-345-678

It should not accept these examples:
1234
123456789
-12345678-
!@@#$%
12ABCBDEF
12-34-56-78

12345678
1234 5678
1234-5678
123-45-678
12-345-678

It should not accept these examples:
1234
123456789
-12345678-
!@@#$%
12ABCBDEF
12-34-56-78