The * symbol can be used with the whitespace character \s to match a string of whitespace characters. The * symbol can be used with the meta character. For example, the regular expression * means: any number of lowercase letters in a row. But if it appears after a character set or class then it finds the repetitions of the whole character set. The regular expression a* means: zero or more repetitions of preceding lowercase character a. The symbol * matches zero or more repetitions of the preceding matcher. These meta characters act differently in different situations. Test the regular expression 2.3 Repetitionsįollowing meta characters +, * or ? are used to specify how many times a subpattern can occur. For example, the regular expression ar means: any character except c, followed by the character a, followed by the letter r. In general, the caret symbol represents the start of the string, but when it is typed after the opening square bracket it negates the character set. Test the regular expression 2.2.1 Negated character set ar => A garage is a good place to park a c ar. The regular expression ar means: a lowercase character a, followed by letter r, followed by a period. he => The car parked in the garage.Ī period inside a character set, however, means a literal period. For example, the regular expression he means: an uppercase T or lowercase t, followed by the letter h, followed by the letter e. The order of the character range inside square brackets doesn’t matter. Use a hyphen inside a character set to specify the characters’ range. Square brackets are used to specify character sets. Test the regular expression 2.2 Character setĬharacter sets are also called character class. ar means: any character, followed by the letter a, followed by the letter r.ar => The car parked in the garage. It will not match return or newline characters. is the simplest example of meta character. The meta characters are as follows:įull stop. Some meta characters have a special meaning and are written inside square brackets. Meta characters do not stand for themselves but instead are interpreted in some special way. Meta characters are the building blocks of the regular expressions. Regular expressions are normally case-sensitive so the regular expression The would not match the string the. The regular expression is matched against an input string by comparing each character in the regular expression to each character in the input string, one after another. The regular expression 123 matches the string 123. For example, the regular expression the means: the letter t, followed by the letter h, followed by the letter e. Table of ContentsĪ regular expression is just a pattern of characters that we use to perform search in a text. It does not match Jo because that string contains uppercase letter and also it is too short. We use the following regular expression to validate a username:Ībove regular expression can accept the strings john_doe, jo-hn_doe and john12_as. We also want to limit the number of characters in username so it does not look ugly. We want to allow the username to contain letters, numbers, underscores and hyphens. Imagine you are writing an application and you want to set the rules for when a user chooses their username. Regular expression is used for replacing a text within a string, validating form, extract a substring from a string based upon a pattern match, and so much more. The word “Regular expression” is a mouthful, you will usually find the term abbreviated as “regex” or “regexp”. What are Regular Expressions?Ī regular expression is a pattern that is matched against a subject string from left to right. In this post, I am going to explain everything that you need to know about regular expressions. As a developer, mastering regular experssions can be a powerful tool to have in your toolbelt you are definitely going to have to use them in your programs at some point of time, search and replace using regular expressions in IDEs comes really handy sometimes and I find myself using them in the command line every single day.ĭifferent applications and programming languages might have different implementations for regular expressions but underlying concepts are mostly the same. Regular expression is a group of characters or symbols which is used to find a specific pattern from some text you can call them wildcards on steroids, if you will.
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