A regex pattern is the string of characters which provides instructions to the regex engine on what is to be matched.
A pattern describes these instructions using assorted punctuation and other symbols known as metacharacters, which can be used alongside normal characters.
Patterns can be used to match specific positions, one or more characters, or a combination of characters and positions.
A pattern can also be called an expression, and within an expression you can have multiple sub-expressions, also known as groups, which can capture a particular segment of text, and/or be used to treating a collection of characters as a single unit.
For full details on the syntax of regex patterns, see the Regex Features section.