What is Regular Expression?
Regular expression is a group of characters or symbols which is used to find a specific pattern from a text.
A regular expression is a pattern that is matched against a subject string from left to right. The word "Regular expression" is a mouthful, you will usually find the term abbreviated as "regex" or "regexp". Regular expression is used for replacing a text within a string, validating form, extract a substring from a string based upon a pattern match, and so much more.
Imagine you are writing an application and you want to set the rules when user choosing their username. We want the username can contains letter, number, underscore and hyphen. We also want to limit the number of characters in username so it does not look ugly. We use the following regular expression to validate a username:
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