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A regular expression (abbreviated regexp or regex) is a way to describe sets of characters using syntactic rules. Many programming languages use or support regular expressions. A regular expression is then used by a special program or part of a programming language. This program will either generate a parser that can be used to match expressions or it will match such expressions itself. A simple use case for this is to find all words or phrases in a text that match a certain pattern. In one case, the 'pattern' might just be a word, but in more complex cases, there might be rules that say that the word needs to start with an uppercase letter, or that only certain letters are allowed.
A regular expression processor is used for processing a regular expression statement in terms of a grammar in a given formal language, and with that examines a text string.
A few examples of what can be matched with regular expressions:
- The sequence of characters "car" appearing consecutively in any context, such as in "car", "cartoon", or "bicarbonate".
- The sequence of characters "car" occurring in that order with other characters between them, such as in "Icelander" or "chandler".
- The word "car" when it appears as an isolated word.
- The word "car" when preceded by the word "blue" or "red".
- The word "car" when not preceded by the word "motor".
- A dollar sign immediately followed by one or more digits, and then optionally a period and exactly two more digits (for example, "$10" or "$245.99"). This does not match "$ 5", because of the space between the dollar sign and the digit, nor "€25", because there is no dollar sign.
Regular expressions can be much more complex than these examples. Many regular expression languages also support "wildcard" characters. A more complex example might be t0 validate a date. Some months have 31 days, others have 30; February has 29 in some cases (leap years). So a use case might be to validate a date.
