Question

I am looking for a solution that can exclusively be done with a regular expression. I know this would be easy with variables, substrings, etc.

And I am looking for PCRE style regex syntax even though I mention vim.

I need to identify strings with 4 numeric digits, and they can’t be all 0’s. So the following strings would be a match:

0001 
1000 
1234 
0101

And this would not:

0000

This is a substring that will occur at a set location within a large string, if that matters; I don’t think it should. For example

xxxxxxxxxxxx0001xxxxx
xxxxxxxxxxxx1000xxxxx
xxxxxxxxxxxx1234xxxxx
xxxxxxxxxxxx0101xxxxx
xxxxxxxxxxxx0101xxxxx
xxxxxxxxxxxx0000xxxxx

Answer 1

 (?<!\d)(?!0000)\d{4}(?!\d)

or, more kindly/maintainably/sanely:

m{
     (?<! \d   )    # current point cannot follow a digit
     (?!  0000 )    # current point must not precede "0000"
     \d{4}          # match four digits at this point, provided...
     (?!  \d   )    # that they are not then followed by another digit
}x