Regular expressions are written in a special language. But a single rule can be applied to any variety of situations. These rules are declarative, which means they are immutable: once declared, they do not change. What are regular expressions, and what is grep?Īs we've noted, a regular expression is a rule used for matching characters in text. This article assumes no prior knowledge of regular expressions, but you should understand how to with the Linux operating system at the command line. The article shows how you can use a regular expression to declare a pattern that you want to match, and outlines the essential building blocks of regular expressions, with many examples. This article examines the basics of using regular expressions under grep. Regular expressions are supported by many programming languages, as well as classic command-line applications such as awk, sed, and grep, which were developed for Unix many decades ago and are now offered on GNU/Linux. You can also apply regular expressions to text that is subject to algorithmic processing at runtime such as content in HTTP requests or event messages. Once mastered, regular expressions provide developers with the ability to locate patterns of text in source code and documentation at design time. In this series, you'll learn more about how the syntax for this and other regular expressions work.Īs just demonstrated, a regex can be a powerful tool for finding text according to a particular pattern in a variety of situations. This example is but one of many uses for regular expressions. *, which matches any block of code text bracketed by tags, to the HTTP request body as part of your search for script injection code. Malicious code can appear in any number of ways, but you know that injected script code will always appear between HTML tags. For example, imagine you need to write code verifying that all content in the body of an HTTP POST request is free of script injection attacks. Use of regular expressions in the real world can get much more complex-and powerful-than that. For instance, using regular expressions, you could find all the instances of the word cat in a document, or all instances of a word that begins with c and ends with t. Perl6) represents an entirely new flavor of Regular Expressions: if you try it out you may be inclined to agree.A regular expression (also called a regex or regexp) is a rule that a computer can use to match characters or groups of characters within a larger body of text. Perl guru Damian Conway states that Raku (i.e. The second line using %% does the same-but also allows a trailing comma. The first line using % detects matches wherein a comma separator is interposed between the pattern to the left. 'modified quantifier') that can be used to solve common regex problems. Now you might be asking yourself, "So what? It looks just like Perl5." That's because the code above is almost a direct translation of Perl5/PCRE. Reading the above regex literally, it says: 'Find one-or-more digits followed by an optional (zero-or-one) dash-one-or-more digits, followed by either a comma or end-of-line ( $$), the entire preceding pattern repeated one-or-more times.' Secondly, modifiers of the basic regex engine like :global acquire a leading colon and appear at the head of the m/./ match construct. Using Raku (formerly known as Perl_6) raku -ne '.put if m:g/^^ ?] ]+ $$/ 'Īn advantage of using Raku is whitespace tolerance within the matcher. It may be written as an (GNU) extended regex: grep -E '^((+(-+)?(,|$))+)$'Īs a Basic Regular Expression (BRE): grep '^\(\(\įor (j = 1 j 2))next for(j=1 j a) next All must be matched, and anything that is not matched gets rejected. That leaves no optional interpretations to the regex machine. If the leading comma should be rejected, use: ^((+(-+)?(,|$))+)$ You may test and edit the PCRE regex in this site It is a very good idea to anchor the regex to the beginning and end of the text tested: ^((^|,)(+(-+)?(,|$))+)$ Then, each of those numbers: 3 (or number ranges: 4-9) should be followed by a comma, (several times): (+(-+)?,)+Įxcept that the last comma might be missing: (+(-+)?(,|$))+Īnd, if required, a leading comma might be present: (^|,)(+(-+)?(,|$))+ Where the ? makes the dash-number sequence optional. Then, a run of digits would be matched by +.Īfter a number (1 or 3 or 26) ther could be a dash '-' followed by one or several digits ( a number again ): +(-+)? Then you would need to write: to be precise. It could match Devanagari numerals, for example. The most basic element to match is a digit, lets assume that, or the simpler \d in PCRE, is a correct regex for a English (ASCII) digit. The full pcre that will match the strings you listed (and those that start with a ,) might be: grep -P '^(+(-+)?(,|$))+$'
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