In
computer science
Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to practical disciplines (includin ...
, a lexical grammar is a
formal grammar
In formal language theory, a grammar (when the context is not given, often called a formal grammar for clarity) describes how to form strings from a language's alphabet that are valid according to the language's syntax. A grammar does not describe ...
defining the
syntax of
tokens. The program is written using characters that are defined by the lexical structure of the language used. The character set is equivalent to the alphabet used by any written language. The lexical grammar lays down the rules governing how a character sequence is divided up into subsequences of characters, each part of which represents an individual token. This is frequently defined in terms of
regular expression
A regular expression (shortened as regex or regexp; sometimes referred to as rational expression) is a sequence of characters that specifies a search pattern in text. Usually such patterns are used by string-searching algorithms for "find" ...
s.
For instance, the lexical grammar for many
programming language
A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language.
The description of a programming l ...
s specifies that a
string literal
A string literal or anonymous string is a string value in the source code of a computer program. Modern programming languages commonly use a quoted sequence of characters, formally " bracketed delimiters", as in x = "foo", where "foo" is a string ...
starts with a character and continues until a matching is found (
escaping makes this more complicated), that an
identifier
An identifier is a name that identifies (that is, labels the identity of) either a unique object or a unique ''class'' of objects, where the "object" or class may be an idea, physical countable object (or class thereof), or physical noncountable ...
is an
alphanumeric
Alphanumericals or alphanumeric characters are a combination of alphabetical and numerical ch