Rename file using Regex

Hello, 

I need to bulk rename files usgin regex. I'm very bad with regex 😕

Here is a simplified sample:

...

$source = "blabla"

$target = "blibli"

$files = Get-ChildItem -file -path ".\Desktop\test" -Recurse

..

foreach ($file in $files)
{

if($($file.Name).ToLower().contains($source)){
echo "$($file.Name) contains $source)"}
else{
echo "$($file.Name) does NOT contain $source)"
}

Now I want to replace the echo lines with something similar to  :

   $NewName = $_.Name -replace "regex syntax..." #I want to manage the fact that my file could be named blabla.txt but also 4141343blablaffjs.txt
   Rename-Item -Path $_.FullName -NewName $NewName

This is where I need your help.

Thank you.