Bash Regex If

If you work with text, you'll appreciate how useful regular expressions are. These are not strictly speaking regular expressions in computing theory but recursive extensions are quickly becoming a part of many libraries. 0 through 4. The next column, "Legend", explains what the element means (or encodes) in the regex syntax. I would like to use patterns from a specific field in one file as regex to search for matching strings in the entire line ($0) of another file. The above metacharacter can be used with various commands to match the string like preg_match, preg_match_all in PHP or used with a "~=" to compare in Perl. Regex Syntax. It's very limited compared to Ruby's regular expressions and is more closely related to shell expansion wildcards. By default, ripgrep will respect your. This regular expression pattern is very useful to implement a strong and complex password. Using Bash's regular expressions Bash has quietly made scripting on Unix systems a lot easier with its own regular expressions. Regex Tester isn't optimized for mobile devices. They can assist in data munging, qualification, categorization, and parsing as well as document editing. This seemingly trivial program is extremely. By default, bash uses basic but R uses extended regular expressions. Regular expression tester with syntax highlighting, PHP / PCRE & JS Support, contextual help, cheat sheet, reference, and searchable community patterns. Bash if statements are very useful. It provides commandline tools for searching and managing packages, and for querying information about them, as well as low-level access to all features of the libapt-pkg library. Bash Performance Test - Regex vs If/Else. So to get all the files ending it. For example, show usage if number of arguments is 0: We might also want to use regular expression to test if $1 is start, stop or restart if $# is no longer 0: But wouldn't it be nice if the tests can be combined…. Regular expressions (Regexp) are special characters which help search data, matching complex patterns. Regular Expression to Match IP Addresses. Regex for "or" of multiple words in grep. Here's what you'd learn in this lesson: To have a search pattern check at the start of the beginning of a string, James shows anchoring within Regular Expressions. [email protected] This is effectively the same as the basic option with the addition of newline '\n' as an alternation separator. RegExr is an online tool to learn, build, & test Regular Expressions (RegEx / RegExp). I didn't really think it would be quite so difficult to find resources that one can use to navigate the command line but I guess most folks use the GUI. However if you are a 'non-computer-savvy' person that won't mean a thing to you. Regular Expressions Tester These are specialized programs that allow you to test your regular expressions against some text. Matched IP addresses can be extracted from a file using grep command. Usually such patterns are used by string searching algorithms for "find" or "find and replace" operations on strings, or for input validation. Basically regular expressions are divided in to 3 types for better understanding. #!/bin/bash read -p "Please enter and confirm number 10 via keyboard : " n. The pattern can be used to search through a text file or listing files and folders. This name comes from its predecessor ed and the specific mode in which you would globally search, using a regular expression, and print the output. Regular Expression Metacharacters. So if you are Bash Scripting or creating a Python program, we can use regex or we can also write a single line search query. In this tutorial we will follow grep -E. This session introduces you to the basics of regular expressions. The "s" means substitute, in the match earlier there is an implied "m" character meaning match. Select-String command to be precise: PS> Get-Help Select-String. Examples, tips and practice activities. Matched IP addresses can be extracted from a file using grep command. …So they're very helpful if you ever need to…help answer a question,…where you know what the answer looks. Regular expressions are a language of their own. Usually such patterns are used by string searching algorithms for "find" or "find and replace" operations on strings, or for input validation. If you want to learn more about regular expressions, the most authoritative source in the filed is the book Mastering Regular Expressions by Jeffrey E. Regular expressions are often simply called regexps or RE, but for consistency I'll be referring to it with its. GitHub Gist: instantly share code, notes, and snippets. The more advanced "extended" regular expressions can sometimes be used with Unix utilities by including the command line flag "-E". However, the syntax is a lot different than the other modern programming languages e. The JSON module contains JSON-specific decode/encode functions that convert a JSON string into a Perl data structure, and vice versa. ' Matches any single character. What can you do with ksh? A heck of a lot! You have access to the full range of UNIX utilities, plus some nifty built-in resources. Regular expressions descend from a fundamental concept in Computer Science called "finite automata theory". This means you're free to copy and share these comics (but not to sell them). These are not strictly speaking regular expressions in computing theory but recursive extensions are quickly becoming a part of many libraries. The manual ("man") pages on Unix systems may be helpful (try "man sed", "man regexp", or the subsection on regular expressions in "man ed"), but man pages are notoriously difficult. hello I want to do a pattern match for string in the if statement, but I am not sure how to use regex inside the if statement. It was written by Ken Thompson more than 45 years ago! The name grep stands for “globally regular expression print”. by importing the module re. IP Addresses consist of two kinds. com - The first Regular Expression Library on the Web!. Use of wildcards is not a regular expression. It was written by Ken Thompson more than 45 years ago! The name grep stands for "globally regular expression print". com> To: robots Subject: The robots mailing list at WebCrawler From: Martijn Koster Date: Thu, 12 Oct 1995 14:39:19 -0700 Sender: owner-robots Precedence: bulk Reply-To: [email protected] Every system administrator has to deal with plain text files on a daily basis. Using Bash's regular expressions Bash has quietly made scripting on Unix systems a lot easier with its own regular expressions. I'm Grant McWilliams, computer science instructor and Linux enthusiast. Regular Expression provides an ability to match a "string of text" in a very flexible and concise manner. We are comparing descriptions on a bank statement and we need to say for one account: If the description matches "BBM TFR TO 20-24-61" then it is this category, otherwise it is another category. POSIX Basic Regular Expressions. Bash if statements are very useful. Use of wildcards is not a regular expression. 0M) id AA20349; Thu, 12 Oct 95 14:39:19 -0700 Message-Id: 9510122139. An input string can match this regular expression even if the two decimal digits that are defined by the second. Bash also have =~ operator which is named as RE-match operator. Lately I’ve been getting familiar with my shell (bash) again, finding new ways to stretch its usefulness as a tool on its own, without pulling in typical auxiliary support from grep, sed, or awk. Browse other questions tagged bash unix regex grep or ask your own question. A number of complex tasks can be solved with simple regular expressions. For example any string made of exactly ten digits matches the regular expression [0-9]{10}, quite useful when you are looking for phone numbers in a given string. 19; win-32 v2018. Regular Expression in Linux/Unix Part 2 This is our second part on Regular Expressions in Linux. blog is like regular expressions, it is not. Regular expressions are implemented in the grep command (and its siblings) in bash (found in Cygwin on Windows and Terminal on OSX) and in grep() and its siblings in R. The "s" means substitute, in the match earlier there is an implied "m" character meaning match. Online regex tester and debugger: PHP, PCRE, Python, Golang and JavaScript Regular Reg Expressions Ex 101. By default, ripgrep will respect your. subdirectories. If you need to parse a JSON-formatted string in Perl, you can use JSON Perl module. There are several ways to do so and so here is the list of ways to rename files in Linux:. – AFH Sep 26 '14 at 11:27. Today, we'll review. So all the files starting with any sign followed bye 44. etc-test--matches directory or filesystem /etc/test. The user can easily perform many types of searching, replacing and report generating tasks by using awk, grep and sed commands. Net Regex v1-4. - AFH Sep 26 '14 at 11:27. ! negates the test, turning it into a "does not match" operator, and a [^] regex character class means "any character other than ". The result is a regular expression that will match a string if a matches its first part and b matches the rest. POSIX regex compiling regcomp() is used to compile a regular expression into a form that is suitable for subsequent regexec() searches. Dissect works well when data is reliably repeated. Backreference Constructs in Regular Expressions. conda install linux-64 v2019. blog is like regular expressions, it is not. Certain commands and utilities commonly used in scripts, such as grep, expr, sed and awk, interpret and use REs. About regular expressions in egrep. I demystify basic and extended regular expressions and use them with Grep, Awk, Sed and Bash's in-process pattern matching. In PowerGREP and EditPad Pro, \b and \B are Perl-style word boundaries, while \y, \Y, \m and \M are Tcl-style word boundaries. About regular expressions in egrep. 21; osx-64 v2019. ripgrep is similar to other popular search. This is the best place to expand your knowledge and get prepared for your next interview. A conditional expression is used with the [[compound command to test attributes of files and to compare strings. Syntax for regex in IF statment. Enjoy the videos and music you love, upload original content, and share it all with friends, family, and the world on YouTube. While there is no native Linux version of RegexBuddy, it is possible to run the Windows version of RegexBuddy on Linux using Wine. Unlike most other languages, spaces are very important when using an if statement. mv old_file. When we run certain commands in Unix/Linux to read or edit text from a string or file, we most times try to filter output to a given section of interest. UNIX & Linux Shell Scripting Tutorial. Tutorial -- Regular Expressions in Rexx (4/2007) Regular expressions are a way to match strings. 0, but I never noticed, probably because most of the machines I'm using are Bash 2. At a high level, regular expressions are a way of defining patterns of text to operate on. You may not like user interfaces of any Linux distribution, but the fact is that all the power of Linux is hidden in it’s CLI(Command Line Interface). grep, awk and sed – three VERY useful command-line utilities Matt Probert, Uni of York grep = global regular expression print In the simplest terms, grep (global regular expression print) will search input files for a search. I'm Grant McWilliams, computer science instructor and Linux enthusiast. Such objects are essentially constructed from a sequence of characters that can include several wildcards and constraints to define the set of rules (the pattern) that make a sequence of characters qualify as a match. Regular expressions are a very powerful tool in Linux. If you consider yourself a good cook, you may or may not know how to make a souffle or baklava. This is where using regular expressions comes in handy. A pattern consists of one or more character literals, operators, or constructs. egrep is an acronym for "Extended Global Regular Expressions Print". Before starting examples we look different commands which provides same functionality. However if you are a 'non-computer-savvy' person that won't mean a thing to you. Meta characters that are. Kris Occhipinti. Below is an example of a regular. Bash Scripting: Learn to use REGEX (Part 2- Intermediate) by Shusain · Published December 20, 2018 · Updated February 26, 2019 In our earlier tutorial, we learned to use regex with some basic concepts & we learned about meta-characters & learned to use those meta-chracters to create some easy but effective regex terms. Hi Bart, Thanks for your feedback – glad you found this useful. 19; win-32 v2018. For an example, see Perform Case-Insensitive Regular Expression Match. A regular expression is a method used in programming for pattern matching. Heinlein, Stranger in a Strange Land. Basically regular expressions are divided in to 3 types for better understanding. Substrings matched by parenthesized subexpressions within the regular expression are saved in the array variable BASH_REMATCH. egrep is an acronym for "Extended Global Regular Expressions Print". ] One of the. I'm Grant McWilliams, computer science instructor and Linux enthusiast. Linux Regular Expressions Fundamentals. The GNU regex library is compliant with POSIX. Bash Else If - Bash elif is used to extend if statement functionality to execute multiple branching conditions. Written By: ph34r A quick cheat sheet for programmers who want to do shell scripting. Linux regular expressions are very much useful, when working with extracting text from files, sorting output from command results, scripting some task etc. There are several ways to do so and so here is the list of ways to rename files in Linux:. Regular expressions and sed & awk Regular expressions Key to powerful, efficient, and flexible text processing by allowing for variable information in the search patterns. An example with the above regex is on github, the file is named "regex_03. From the bash man page: [] An additional binary operator, =~, is available, with the same precedence as == and !=. Browse other questions tagged bash regex or ask your own question. To search for two strings in one line of text you will need to combine two regular expressions – in your case, one for the IP addresses and one for the date. And while I'm comparing glob patterns to regular expressions, there's an important point to be made that may not be immediately obvious: glob patterns are just another syntax for doing pattern matching in general in bash. Basic regular expressions: This set includes very basic set of regular. Yes, I realize that is one big regex (note that the lines should all really be concatenated, but I broke it apart at the | points to make it easier to read). A RegEx, or Regular Expression, is a sequence of characters that forms a search pattern. RegEx with sed (bash script) submitted 2 years ago * by Patrick_the_Saint I am working on a script to parse (excuse me if this is the wrong term) text and remove bits and pieces of it. 1, Tcl was enhanced to support extended regular expressions to the POSIX standard. subdirectories. RegEx can be used to check if a string contains the specified search pattern. In Python, we can use regular expressions to find, search, replace, etc. by importing the module re. You can create an OR like statement, using | character where a regular expression is made up of two or more regular expressions, say, A and B. Linux Fu: Regular Expressions. Hi, I know that I should use =~ to match regex (bash version 4). If you wanted to eliminate "abc" from the output, you must expand the regular expression to match the rest of the line and explicitly exclude part of the expression using "(", ")" and "\1", which is the next topic. The key question is whether this awk command is really useful. One can test that a bash variable starts with a string or character in bash efficiently using any one of the following. In the previous post, we talked about bash functions and how to use them from the command line directly and we saw some other cool stuff. A regular expression (or regex) is simply text that contains special characters that together defines a pattern that can be used to match text, in this case filenames. Understanding Regular Expressions Regular Expressions are a feature of UNIX. Generally speaking, UNIX scripting is a matter of using the various command line utilities as appropriate, with the shell as a means of facilitating communication between each step. egrep: Use the regular expression grammar used by the grep utility, with the -E option, in POSIX. Regular Expressions Cheat Sheet from DaveChild. Interval Regular expressions These are used to mention no of character/character set reputation info. 8 (Regular Expression Notation). Whole combination is means, 6 to 20 characters string with at least one digit, one upper case letter, one lower case letter and one special symbol (“@#$%”). Bash Programming Cheat Sheet. These are "interesting" regex or globbing examples. Enjoy the videos and music you love, upload original content, and share it all with friends, family, and the world on YouTube. AWK is very powerful and efficient in handling regular expressions. Many utility tools exist in the Linux operating system to search and generate a report from text data or file. regcomp() is supplied with preg, a pointer to a pattern buffer storage area; regex, a pointer to the null-terminated string and cflags, flags used. Perl is the Swiss Army chainsaw of scripting languages: powerful and adaptable. …Regular expressions let you specify a pattern…to look for in text,…rather than searching for an…explicit string of specific characters. Regex for sed to grab multiple lines or a better way? Ask Question Asked 8 years ago. A must-have reference for every DBA running or planning to run Oracle on a Linux platform. If you're still leaning on grep and sed commands to get your scripts. The following illustrates the basic syntax of regular expression. If you bash script then it is a good idea to make sure it does not matter where you put the script and what is your current working directory when you run it. A regular expression, regex or regexp (sometimes called a rational expression) is a sequence of characters that define a search pattern. 20 awk examples. hello I want to do a pattern match for string in the if statement, but I am not sure how to use regex inside the if statement. To represent this, we use a similar expression that excludes specific characters using the square brackets and the ^ (hat). Use of wildcards is not a regular expression. Regular Expression provides an ability to match a "string of text" in a very flexible and concise manner. A plain string is a regular expression that matches the string exactly. 0) As one passes from the stages of beginner to intermediate of Regular Expression Parsing, the if statement (? () | ) really opens up what one can do with Regular Expressions. They are an important tool in a wide variety of computing applications, from programming languages like Java and Perl, to text processing tools like grep, sed, and the text editor vim. Bash regular expression match with groups including example to parse http_proxy environment variable - bash_regex_match_groups. Variables · Functions · Interpolation · Brace expansions · Loops · Conditional execution · Command substitution · One-page guide to Bash scripting. C/C++, Java, Python, Javascript. For the most part, they are fairly straight forward so this will be a quick rundown on how to use each and any neat features they might have. grep, awk and sed – three VERY useful command-line utilities Matt Probert, Uni of York grep = global regular expression print In the simplest terms, grep (global regular expression print) will search input files for a search. A number of complex tasks can be solved with simple regular expressions. When working with regular expressions in a shell script the norm is to use grep or sed or some other external command/program. Actually, it is older than Linux itself. Java Regular Expression Example. " For this, see "Mastering Regular Expressions" by Jeffrey Friedl (O'Reilly, 1997). This is the best place to expand your knowledge and get prepared for your next interview. 2, section 2. I am looking for someth | The UNIX and Linux Forums. Hi, I need a regular expression that does not match a pattern. A string either matches a regular expression or it doesn't - there is no approximate match. Since version 3 of bash (released in 2004) there is another option: bash's built-in regular expression comparison operator "=~". Shell patterns. A regular expression is a pattern that the regular expression engine attempts to match in input text. As we all know we can use conditional statements in BASH. I'm Grant McWilliams, computer science instructor and Linux enthusiast. But there are certain. Regular Expressions Cheat Sheet from DaveChild. March 9, 2018. In this tutorial we will look =~ operator and use cases. Bash regular expression match with groups including example to parse http_proxy environment variable - bash_regex_match_groups. Bash if statements are very useful. The Linux operating system makes use of terminal commands to work with a computer's file system. Use this method when a script has to perform a slightly different function depending on the values of the input parameters, also called arguments. They describe a pattern to match, a sequence of characters, not words, within a line of text. Before we start, let us ensure we have a local copy of /etc/passwd text file to work with sed. Regular Expression to Given a list of strings (words or other characters), only return the strings that do not match. Meta characters that are. Different Regular Expression Engines A regular expression "engine" is a piece of software that can process regular expressions, trying to match the pattern to the given string. Regex for “or” of multiple words in grep. If statements (and, closely related, case statements) allow us to make decisions in our Bash scripts. Use of wildcards is not a regular expression. Level up your coding skills and quickly land a job. This seemingly trivial program is extremely. These are not strictly speaking regular expressions in computing theory but recursive extensions are quickly becoming a part of many libraries. I also do multi-line searches and non-greedy matches using Perl compatible regular expressions with Grep. If statements (and, closely related, case statements) allow us to make decisions in our Bash scripts. And you can use them in a number of different places: After the == in a bash [[ expr ]] expression. 0, using the =~ operator, just like Perl. +?)\1+$ regular expression can match non-prime numbers (in their unary form). #!/bin/bash read -p "Please enter and confirm number 10 via keyboard : " n. In bash, when to use glob pattern and when to use regular expression? Thanks! Regex are more versatile and "convenient" than "glob patterns", however unless you are doing complex tasks that "globbing/extended globbing" cannot provide easily, then there's no need to use regex. : m: For patterns that include anchors (i. Please have a look at our first part here. The combination of parameter expansion and regex operators can make bash regular expression syntax "almost readable", but there are still some gotchas. A regular expression is a pattern that describes a set of strings. A regular expression, regex or regexp (sometimes called a rational expression) is a sequence of characters that define a search pattern. May 05, 2013, 8:37 PM — Since version 3 (circa 2004), bash has a built-in regular expression comparison operator, represented by =~. A number of complex tasks can be solved with simple regular expressions. Written by Zeph Grunschlag. However, learning Regular Expressions and effectively implementing them in your daily work will doubtlessly reward your learning effort by greater work efficiency and time savings. A pattern consists of one or more character literals, operators, or constructs. everything the regular expression did not match. May 05, 2013, 8:37 PM — Since version 3 (circa 2004), bash has a built-in regular expression comparison operator, represented by =~. Variables · Functions · Interpolation · Brace expansions · Loops · Conditional execution · Command substitution · One-page guide to Bash scripting. Here is a quick summary of the special characters used in the grep tool and their meaning: Text version. When attempting to build a logical “and” operation using regular expressions, we have a few approaches to follow. For example, if a pair of back-ticks in the regexp is not quoted, it would result in the text between the back-ticks being executed as a Bash sub-process — and if this happens to be a valid command, the text returned by it takes the regular expression's place in the command-line parameters given to Grep!. Really, what I did was a little more complicated. This section of the Linux tutorial teaches the basics of regular expressions and grep on the Linux command line. Linux Regular Expression. If you use bash for scripting you will undoubtedly have to use conditions a lot, for example for an if … then construct or a while loop. The "Anchors, Groups, and sed" Lesson is part of the full, Introduction to Bash, VIM & Regex course featured in this preview video. A regular expression is a pattern that describes a set of strings. In bash, when to use glob pattern and when to use regular expression? Thanks! Regex are more versatile and "convenient" than "glob patterns", however unless you are doing complex tasks that "globbing/extended globbing" cannot provide easily, then there's no need to use regex. However, the man page is not very clear. The matching should cover the entire input string (not partial). Actually, it is older than Linux itself. 2 regular expressions DESCRIPTION top Regular expressions ("RE"s), as defined in POSIX. Of course, there have been a certain number of shell scripts written specifically for Linux systems, some of which incorrectly stated that they could run with /bin/sh when in fact they required /bin/bash, and these scripts will have broken due to this change. 10-digit phone number with hyphens match whole word Find Substring within a string that begins and ends with paranthesis Simple date dd/mm/yyyy all. If the operating system on which Bash is running provides these special files, Bash will use them; otherwise it will emulate them internally with this behavior: If the file argument to one of the primaries is of the form /dev/fd/N, then file descriptor N is checked. Batch RegEx Free 5. Your question is confusing: you can either have an exact match (stings are identical) or you can have a regex match (string matches the template in a regular expression). Basic regular expressions: This set includes very basic set of regular. Written by Zeph Grunschlag. For a Unix administrator, the regular expression is a life saver. The find command is a powerful *nix utility that allows the user to find files located in the file system via criteria such as the file name, when file was last accessed, when the file status was last changed, the file's permissions, owner, group, size, or even number of inodes. #!/bin/bash read -p "Please enter and confirm number 10 via keyboard : " n. (Aren't there always?). Bash is very flexible, and has many advanced features that you won't see in batch scripts. Learn the fundamentals of Bash regular expressions for the purpose of matching patterns. : zzcidade # mostra uma cidade qualquer - zzcidade campos # mostra as cidades com "Campos" no nome - zzcidade '(SE)' # mostra todas as cidades de Sergipe - zzcidade ^X # mostra as cidades que começam com X - -. An input string can match this regular expression even if the two decimal digits that are defined by the second. Basic regular expressions: This set includes very basic set of regular. Java Regular Expression Example. Extended Regular Expressions were added to Bash version 3 and made more robust in later versions. They can be used with a variety of programs like bash, vi, rename, grep, sed, and more. This is not intended to teach programming, etc. I'm Grant McWilliams, computer science instructor and Linux enthusiast. Usually a “d” would delete the entire line of a match, but adding the “!” causes it only to delete lines that do not match. When working with regular expressions in a shell script the norm is to use grep or sed or some other external command/program. May 05, 2013, 8:37 PM — Since version 3 (circa 2004), bash has a built-in regular expression comparison operator, represented by =~. RegEx can be used to check if a string contains the specified search pattern. Bash is very flexible, and has many advanced features that you won't see in batch scripts. However if you are a 'non-computer-savvy' person that won't mean a thing to you. Test your regex by visualizing it with a live editor. Every sequential character in a regular expression is “and’ed” together. If you use bash for scripting you will undoubtedly have to use conditions a lot, for example for an if … then construct or a while loop. Linux Fu: Regular Expressions. This section of the Linux tutorial teaches the basics of regular expressions and grep on the Linux command line. So if you are Bash Scripting or creating a Python program, we can use regex or we can also write a single line search query. However if you are a 'non-computer-savvy' person that won't mean a thing to you. egrep: Use the regular expression grammar used by the grep utility, with the -E option, in POSIX. A number of complex tasks can be solved with simple regular expressions. Use the regular expression grammar used by the grep utility in POSIX. Backreference Constructs in Regular Expressions. I'm Grant McWilliams, computer science instructor and Linux enthusiast. Shell case statement is similar to those in Pascal and switch statement in C. Bash's regular expression comparison. It can be used to test a variable against set of patterns. To require the match to occur only at the beginning or end, use an anchor. I would like to use patterns from a specific field in one file as regex to search for matching strings in the entire line ($0) of another file. A pattern consists of one or more character literals, operators, or constructs. RegEx Testing From Dan's Tools. PCRE is supported by some tools (like grep -P) under some conditions, but the bash regex support inside the [[…]] idiom is only for extended regex (like grep -E). Regular expressions (shortened as "regex") are special strings representing a pattern to be matched in a search operation. Learn vocabulary, terms, and more with flashcards, games, and other study tools. One of these standards defines two flavors of regular expressions. Regular expressions (regex) are similar to Glob Patterns, but they can only be used for pattern matching, not for filename matching. regex API for pattern matching with regular expressions. With respect to the C library, the primary focus is the GNU C library (), although, where known, documentation of variations on other C libraries available for Linux is also included. Hi, I know that I should use =~ to match regex (bash version 4). From LibreOffice Help Search interprets the special character that follows the "\" as a normal character and not as a regular. Linux Regular Expressions Regular expression meta characters. Regex can be used in a variety of programs like grep, sed, vi, bash, rename and many more. "alexa?" Some characters have special meanings in a regular expression. directories or objects with a similar name. Since version 3 of bash (released in 2004) there is another option: bash's built-in regular expression comparison operator "=~". It is a very powerful tool in Linux. Note: s could be empty and contains only lowercase letters a-z. In this article you'll find a regular expressions themselves and an example of how to extract matched IP addresses from a file with the grep command. A regular expression (or regex) is simply text that contains special characters that together defines a pattern that can be used to match text, in this case filenames. - [Narrator] Regular expressions…are a way of describing text…and they're used very widely…by programmers and system administrators. Regular expressions (Regexp) are special characters which help search data, matching complex patterns. The first is a. A regular expression is a pattern that provides a flexible and concise mean to match the string of text. The GNU regex library is compliant with POSIX.