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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 .
A string-searching algorithm, sometimes called string-matching algorithm, is an algorithm that searches a body of text for portions that match by pattern. A basic example of string searching is when the pattern and the searched text are arrays of elements of an alphabet ( finite set ) Σ.
Returns the position of the start of the first occurrence of substring in string. If the substring is not found most of these routines return an invalid index value – -1 where indexes are 0-based, 0 where they are 1-based – or some value to be interpreted as Boolean FALSE. Related instrrev
Below is a simple grammar, defined using the notation of regular expressions and Extended Backus–Naur form. It describes the syntax of S-expressions, a data syntax of the programming language Lisp, which defines productions for the syntactic categories expression, atom, number, symbol, and list:
Regexp searches are restricted on the server, so this template reduces the regex search footprint by using the prefix: filter every time, restricting the search domain to a namespace at most. The prefix: parameter can further filter a namespace by specifying pagenames that start with a given letter(s).
The latter may allow its elements to be mutated and the length changed, or it may be fixed (after creation). A string is generally considered as a data type and is often implemented as an array data structure of bytes (or words ) that stores a sequence of elements, typically characters, using some character encoding .
The second condition says that given each character of string w, the machine will transition from state to state according to the transition function δ. The last condition says that the machine accepts w if the last input of w causes the machine to halt in one of the accepting states. Otherwise, it is said that the automaton rejects the string.
Many field values may contain a quality (q) key-value pair separated by equals sign, specifying a weight to use in content negotiation. [9] For example, a browser may indicate that it accepts information in German or English, with German as preferred by setting the q value for de higher than that of en, as follows: Accept-Language: de; q=1.0 ...