Regular Expression is a text pattern that includes ordinary characters (for example, letters between a and z) and special characters (called "metacharacters").
Regular expressions use a single string to describe and match a series of strings that match a certain syntax rule.
Regular expression - matching rules syntax
Everything starts from the basics. Patterns are the most basic elements of regular expressions. They are a set of characters that describe the characteristics of a string. Patterns can be simple, consisting of ordinary strings, or very complex, often using special characters to represent a range of characters, recurrences, or to represent context.
Regular expression - matching rules example
^once