What does this regex mean and why

Posted by Kalec on Ask Ubuntu See other posts from Ask Ubuntu or by Kalec
Published on 2012-06-05T16:17:22Z Indexed on 2012/06/05 16:48 UTC
Read the original article Hit count: 868

Filed under:

regex

|

sed

$ sed "s/\(^[a-z,0-9]*\)\(.*\)\( [a-z,0-9]*$\)/\1\2 \1/g" desired_file_name

I apreciate it even if you only explain part of it or at lest structure it with words as in s\alphanumerical_at_start\something\alphanumerical_at_end\something_else\global

Could someone explain what that means, why and are all regEx so ... awful ?

I know that it replaces the first lowcase alphanumerical word with the last one. But could you explain bit by bit what's going on here ? what's with all the /\ and \(.*\)\ and everything else ?

I'm just lost.

EDIT: Here is what I do get: (^[a-z0-9]*) starting with a trough z and 0 trough 9; and [a-z,0-9]*$ is the same but the last word (however [0-9,a-z] = just first 2 characters / first character, or the entire word ?). Also: what does the * or the \(.*\)\ even mean ?

© Ask Ubuntu or respective owner

Related posts about regex

Find multiple regex in each line and skip result if one of the regex doesn't match

as seen on Stack Overflow - Search for 'Stack Overflow'
I have a list of variables: variables = ['VariableA', 'VariableB','VariableC'] which I'm going to search for, line by line ifile = open("temp.txt",'r') d = {} match = zeros(len(variables)) for line in ifile: emptyCells=0 for i in range(len(variables)): regex = r'('+variables[i]+r')[:|=|\(](-… >>> More
OWASP Regex Repository: Is this regex correct?

as seen on Stack Overflow - Search for 'Stack Overflow'
I was looking at the regular expression for validating various data types from the (OWASP Regex Repository). One of the regular expressions in there is called safetext and looks like: ^[a-zA-Z0-9\s.\-]+$ My first question is: Is this regular expression correct? complementary question If this… >>> More
Make a Perl-style regex interpreter behave like a basic or extended regex interpreter

as seen on Stack Overflow - Search for 'Stack Overflow'
I am writing a tool to help students learn regular expressions. I will probably be writing it in Java. The idea is this: the student types in a regular expression and the tool shows which parts of a text will get matched by the regex. Simple enough. But I want to support several different regex… >>> More
JS regex isn't matching, even thought it works with a regex tester

as seen on Stack Overflow - Search for 'Stack Overflow'
I'm writing a piece of client-side javascript code that takes a function and finds the derivative of it, however, the regex that's supposed to match with the power rule fails to work in the context of the javascript program, even though it sucessfully matches when it's used with an independent regex… >>> More
c# RegEx with "|"

as seen on Stack Overflow - Search for 'Stack Overflow'
I need to be able to check for a pattern with | in them. For example an expression like d*|*t should return true for a string like "dtest|test". I'm no regex hero so I just tried a couple of things, like: Regex Pattern = new Regex("s*\|*d"); //unable to build because of single backslash Regex Pattern… >>> More

Related posts about sed

sed + RE , want to ignore remarked line using sed

as seen on Stack Overflow - Search for 'Stack Overflow'
I want to replace the a string with the word string as the following example down , but if a string exist also after comment then a string will replaced too In which way I can add to the sed command the irregular exp that ignore from the #. [root@localhost tmp]# more test a b aa bb #a #b #aa #bb [root@localhost… >>> More
script to search and replace deprecated functions

as seen on Stack Overflow - Search for 'Stack Overflow'
Hi, I am using the following script to search and replace the deprecated functions in a file with the newer ones. 5 for strFile in `ls deprecated_functions_search_and_replace.txt ` 6 do 7 sed "s/ereg_replace[^\(]*(\([^,]*\),/preg_replace\1('#'.\2.'#',/g" $strFile > temp_file 8 … >>> More
Footer not stretching 100% when horizontally scrolled

as seen on Pro Webmasters - Search for 'Pro Webmasters'
I have a footer which is set to 100% width, but if i size the window smaller so a horizontal scrollbar appears, using the scrollbar shows whitespace to the right of the footer ... its not spanned 100% of the page, just the viewport. <!doctype html> <html lang="en" class="no-js"> <head> … >>> More
What does this regex mean and why

as seen on Ask Ubuntu - Search for 'Ask Ubuntu'
$ sed "s/\(^[a-z,0-9]*\)\(.*\)\( [a-z,0-9]*$\)/\1\2 \1/g" desired_file_name I apreciate it even if you only explain part of it or at lest structure it with words as in s\alphanumerical_at_start\something\alphanumerical_at_end\something_else\global Could someone explain what that means, why and… >>> More
Sed problem in a Bash script

as seen on Ask Ubuntu - Search for 'Ask Ubuntu'
Hello there. I'm having a problem using the sed command . I'm trying to write a bash script that does the following : search for the line that contain :@ then save the line that contained :@ and replace it with new line as in the following: #! /bin/bash echo "Please enter the ip address… >>> More