Raw string

A string literal or anonymous string is a type of literal in programming for the representation of a string value within the source code of a computer program. Most often in modern languages this is a quoted sequence of characters (formally "bracketed delimiters"), as in x = "foo", where "foo" is a string literal with value foo – the quotes are not part of the value, and one must use a method such as escape sequences to avoid the problem of delimiter collision and allow the delimiters themselves to be embedded in a string. However, there are numerous alternate notations for specifying string literals, particularly more complicated cases, and the exact notation depends on the individual programming language in question. Nevertheless, there are some general guidelines that most modern programming languages follow.

Most modern programming languages use bracket delimiters (also balanced delimiters) to specify string literals. Double quotations are the most common quoting delimiters used:

An empty string is literally written by a pair of quotes with no character at all in between:

Some languages either allow or mandate the use of single quotations instead of double quotations (the string must begin and end with the same kind of quotation mark and the type of quotation mark may give slightly different semantics):

Note that these quotation marks are unpaired (the same character is used as an opener and a closer), which is a hangover from the typewriter technology which was the precursor of the earliest computer input and output devices.

In terms of regular expressions, a basic quoted string literal is given as:

This means that a string literal is written as: a quote, followed by zero, one, or more non-quote characters, followed by a quote. In practice this is often complicated by escaping, other delimiters, and excluding newlines.

...
Wikipedia