REGEX is short for regular expression. A regular expression is way to search for a sequence of characters or a character pattern in a body of text. REGEX uses a search pattern that can be difficult to understand but can find multiple instances of a character pattern throughout a text document.
For example, if I want to find a phone number in a body of text (such as a markdown document or javascript code or simply an article or word document), I can use a REGEX search pattern.
The following REGEX search pattern will find all phone numbers (e.g. 123-456-7899) that match this pattern of 3 digits followed by a "-" followed by 3 digits followed by a "-" followed by 4 digits. The REGEX to do so is
REGEX = "\d{3}-\d{3}-\d{4f}"