In the case of ] we are looking for a single space or tab character It can be used within a bracketed expression and has special meaning. is an example of a POSIX Character class.The Plus sign ( ) in this context will match any consecutive characters that are described by the bracketed expression (lower case letters a through z).If the string we are testing contains any of the characters listed in the range specified in the bracketed expression ( any lower case letter between a and z), this symbol is considered to be matched. # Example 4: Bracketed Expressions and Character classes This causes the regex check to fail.Įxample 4: Bracketed expressions and Character Classes We altered the pattern to look for two digits in a row where our sample string only has one digit.Regex will only match (return true) if it can find text that exactly matches its pattern.For more complete information on Bracketed Expressions, see the Notes section above.Single characters and ranges of characters are accepted as well (as we'll demonstrate below). You don't have to limit the bracketed expression to numbers.In this case we wanted to find out if the string we are testing meets our parameters for acceptability ( are there single digit numbers in expected places in the string?).Bracketed expressions can be used to define a range of possible characters.# Example 2: Use regex to check for a string that could vary a bit BASH_REMATCH shows the part of the text string that matched regex (if there is a match)Įxample 2: Check for a string which can vary slightly.Regex testing is only available when used within the double bracket expression ].The =~ operator is used for regular expression testing in bash conditionals.Here's a screen capture of the script output: # Perform regex check and determine match / no match # Regular expression we'll use to analyze the stringĮcho "This is the regex we will use to try analyzing the text:" Var="this is A text9 strin7g with numb3rs in it"Įcho "this is the string we are going to analyze:" # This script demonstrates regular expressions in bash I'll be showing examples and explaining them. Regular Expressions in Bash Conditional statementsīash allows you to compare Literal strings and variables against regular expressions when you use the [[ ( double bracket) and =~ (regex comparator, equal sign tilde) in your if statements. If I were to try and cover the entire scope of Regex in a single article it would probably kill me. In this article I'm focusing on a particular use of Regular expression here. For more details, see the list of references above. This dialect is part of the POSIX Specification. See, wikipedia and the rest of the references in the Notes section (above) for more detailed explanations on what Regex is and is not.īash uses a dialect of Regular expression which is different from Powershell /. Using symbols you can specify exactly what you are looking for in text strings of any length. BASH_REMATCH (google books archive of O'reilly Bash cookbook)Ī Regular Expression is a type of search query for text.Cover the Bash 3.2 Syntax around regex in conditionals.Briefly cover what a regular expression is, and where you can go to learn more.They come in handy when you need to analyze the output of CLI Commands. Regular Expressions are powerful, flexible and precise. It's a range, which in ASCII, includes \, [, ^, and _.I've come to rely on Regex for most of my text analysis needs. Of course, even though it didn't error, it isn't doing what you want: $ echo '\^_' | LC_ALL=C egrep '^ $' Ranges are very collating-sequence-dependent, and portable programs should avoid relying on them.įor example: $ echo '\_' | LC_ALL=en_US.UTF8 egrep '' This should work regardless of your libc version (in either egrep or bash).Įdit: This actually depends on your locale settings too. You need to put your - at the end, like or: echo "username : username usergroup" | egrep "^( : ) (usergroup)$" Since \ is not special, that is a range, just like would be. Here is a simpler version, that also gives an error: $ echo 'hi' | egrep '' Trying the regexp with egrep gives an error: $ echo "username : username usergroup" | egrep "^( : ) (usergroup)$" ll other special characters, including '\', lose their special significance within a bracket expression. … To include a literal '-', make it the first or last character…. A bracket expression is a list of characters enclosed in "".
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