Smart answer:

Your search for What is a regex returned the following results:

a sequence of characters that define a search pattern' That define a search pattern

A Regular Expression (aka regex) is 'a sequence of characters that define a search pattern' That define a search pattern.

Source:

an organization of assertions that either succeed or fail

Notionally, a regex is an organization of assertions that either succeed or fail.

Source:

a text string that describes a pattern that a regex engine uses in order to find text (or positions) in a body of text, typically for the purposes of validating, finding, replacing or splitting

A regex is a text string that describes a pattern that a regex engine uses in order to find text (or positions) in a body of text, typically for the purposes of validating, finding, replacing or splitting.

Source:

a pattern that the regular expression engine attempts to match in input text

A regex (regular expression) is a pattern that the regular expression engine attempts to match in input text.

Source:

a pattern matching strings within other strings

A Regex (Regular Expression) is basically a pattern matching strings within other strings.

Source:

an expression which searches text looking for a match

A RegEx is an expression which searches text looking for a match.

a string inside forward slashes

A regex is a string inside forward slashes.

Source:

a text string that creates search patterns within a statement

A regular expression (also known as a regex) is a text string that creates search patterns within a statement.

Source:

languages) to indicated the next expressions

languages) to indicated the next expressions is a regex.

a computing term for a regular expression

A REGEX is a computing term for a regular expression.