Added TIL on prime numbers and regular expressions

A small entertaining part I picked up on Youtube

README.md

@@ -783,6 +783,7 @@
 ### Regexp - regular expressions
 
 - [Regex Posix](regexp/posix.md)
+- [Prime Numbers](regexp/prime_numbers.md)
 
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 ### Renovate - a bot for automating dependency updates

regexp/prime_numbers.md

@@ -0,0 +1,27 @@
+# Prime Numbers
+
+I fell over a video on YouTube demonstrating a regular expression that could check if a numbers was a prime.
+
+The video is entitled: "How on Earth does ^.?$|^(..+?)\1+$ produce primes?", well it does not really produce primes it just checks them anyway, fascinating video.
+
+This is the regex:
+
+```regex
+^.?$|^(..+?)\1+$
+```
+
+And it Perl form:
+
+```regex
+^         # beginning of line of line
+.?
+$         # end of line
+|         # or
+^         # beginning of line of line
+(..+?)\1+
+$         # end of line
+```
+
+## Resources and References
+
+- [YouTube: @standupmaths: "How on Earth does ^.?$|^(..+?)\1+$ produce primes?"](https://youtu.be/5vbk0TwkokM?si=_lNEsO8ZU21MJhG6)