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Some classes of regular languages can only be described by deterministic finite automata whose size grows exponentially in the size of the shortest equivalent regular expressions. The standard example here is the languages L k consisting of all strings over the alphabet {a,b} whose kth-from-last letter equals a.
The following table lists many common symbols, together with their name, how they should be read out loud, and the related field of mathematics. Additionally, the subsequent columns contains an informal explanation, a short example, the Unicode location, the name for use in HTML documents, [1] and the LaTeX symbol.
Regex experts should note that \n does not mean "newline," \d does not mean "digit," and so on: In MediaWiki syntax, the only use of \ is to escape metacharacters. / is special because it indicates the end of the regex. For example, insource:/yes/no/ is treated the same as insource:/yes/ no (because the keyword search for no/ ignores
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).
In mathematics, it is more commonly known as the free monoid construction. The application of the Kleene star to a set V {\displaystyle V} is written as V ∗ {\displaystyle V^{*}} . It is widely used for regular expressions , which is the context in which it was introduced by Stephen Kleene to characterize certain automata , where it means ...
There are many variants and extensions of BNF, generally either for the sake of simplicity and succinctness, or to adapt it to a specific application. One common feature of many variants is the use of regular expression repetition operators such as * and +. The extended Backus–Naur form (EBNF) is a common one.
1. Between two numbers, either it is used instead of ≈ to mean "approximatively equal", or it means "has the same order of magnitude as". 2. Denotes the asymptotic equivalence of two functions or sequences. 3. Often used for denoting other types of similarity, for example, matrix similarity or similarity of geometric shapes. 4.
This example uses some of the following regular expression metacharacters (sed supports the full range of regular expressions): The caret (^) matches the beginning of the line. The dollar sign ($) matches the end of the line. The asterisk (*) matches zero or more occurrences of the previous character.