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A regular expression (shortened as regex or regexp), [1] sometimes referred to as rational expression, [2] [3] is a sequence of characters that specifies a match pattern in text. Usually such patterns are used by string-searching algorithms for "find" or "find and replace" operations on strings , or for input validation .
For example, insource:/".*"/ means the same thing as insource:/\.\*/. The character # is also a metacharacter and must be escaped. [clarification needed] Regex experts should note that \n does not mean "newline," \d does not mean "digit," and so on. Regex experts should note that ^ does not mean "beginning of text" and $ does not mean "end of ...
In theoretical computer science and formal language theory, a regular language (also called a rational language) [1] [2] is a formal language that can be defined by a regular expression, in the strict sense in theoretical computer science (as opposed to many modern regular expression engines, which are augmented with features that allow the recognition of non-regular languages).
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Given a set of strings (also called "positive examples"), the task of regular language induction is to come up with a regular expression that denotes a set containing all of them. As an example, given {1, 10, 100}, a "natural" description could be the regular expression 1⋅0 * , corresponding to the informal characterization " a 1 followed by ...
Some authors call this type of grammar a right-regular grammar (or right-linear grammar) [1] and the type above a strictly right-regular grammar (or strictly right-linear grammar). [2] An extended left-regular grammar is one in which all rules obey one of A → w, where A is a non-terminal in N and w is in Σ * A → Bw, where A and B are in N ...
Tools have been written that automatically generate a lexer from a lexical specification written in regular expressions and a parser from the phrase grammar written in BNF: this allows one to use declarative programming, rather than need to have procedural or functional programming. A notable example is the lex-yacc pair.
An extended context-free grammar (or regular right part grammar) is one in which the right-hand side of the production rules is allowed to be a regular expression over the grammar's terminals and nonterminals. Extended context-free grammars describe exactly the context-free languages.