Answer 1

A regular expression is a pattern of characters.
The pattern is used to do pattern-matching "search-and-replace" functions on text.

Java:
https://www.w3schools.com/java/java_regex.asp

HTML:
https://www.w3schools.com/tags/att_input_pattern.asp

Useful for testing patterns as you make them:
https://regex101.com

?:
https://github.com/ziishaned/learn-regex