Select-String: match a string only if it isn't preceded by * character

I have this line in a Powershell script:

if (Select-String -Path "$holidays" -Pattern "(?<!\*)$datestr" -SimpleMatch -Quiet)

$holidays is a text file where i have some date:

2023-09-04
2023-11-23
*2023-11-24
2023-12-25

$datestr is today date: 2023-11-23

why negative lookbehind pattern doesn't work in the line above?

I just want to esclude (don't match) all dates that start with *

I think that something is wrong in negative lookbehind because the line:

if (Select-String -Path "$holidays" -Pattern $datestr -SimpleMatch -Quiet)

works perfectly --> results is true

in the first line results is false

As explained in comments, when -SimpleMatch is used the cmdlet doesn't use regex:

Indicates that the cmdlet uses a simple match rather than a regular expression match. In a simple match, Select-String searches the input for the text in the Pattern parameter. It doesn't interpret the value of the Pattern parameter as a regular expression statement.

In your case the cmdlet is looking for a literal (?<!\*)2023-11-23 instead of 2023-11-23 not preceded by *:

$datestr = '2023-11-23'
'2023-11-23' | Select-String -Pattern "(?<!\*)$datestr" -SimpleMatch -Quiet # No output
'2023-11-23' | Select-String -Pattern "(?<!\*)$datestr" -Quiet              # True

In summary, your condition should be:

if (Select-String -Path $holidays -Pattern "(?<!\*)$datestr" -Quiet) {
    # do stuff
}