Login or Register to Ask a Question and Join Our Community


UNIX for Dummies Questions & Answers

Need help with Regex for bash

 
Thread Tools Search this Thread
Top Forums UNIX for Dummies Questions & Answers Need help with Regex for bash
# 1  
Old 12-30-2012
Need help with Regex for bash
Hi, I am trying to match this word: hexagon-bx.mydomain.com with regex. I have tried this:

Code:
"[a-z0-9\-\[bu]\.*]*$"

So far I have not been successful. I also need to make sure that the regex will match words that just have lowercase letters and numbers in them, such as camera01. How can I create such an expression? I am trying to do a expr match on "[a-z0-9\-\[bu]\.*]*$" so that it would pick up words like hexagon-bx/mydomain.com. Any help is greatly appreciated!
Last edited by radoulov; 12-30-2012 at 09:09 AM..
# 2  
Old 12-30-2012
What you proposed will match your requirement. But it will match almost anything else that does not have an upper case or punctuation char at the end. What do you want to discriminate it against? Pls post input and desired output, and the command that you want to run.
Last edited by RudiC; 12-30-2012 at 10:03 AM.. Reason: typo
# 3  
Old 12-30-2012
A regex that would do that can range from simple and broad to complicated and precise. It depends on the requirements. Let's start with simple:
Code:
^[a-z0-9.-]*$

To also match the slash:
Code:
^[/a-z0-9.-]*$

Last edited by Scrutinizer; 12-30-2012 at 08:46 AM..
This User Gave Thanks to Scrutinizer For This Post:
newbie2010
# 4  
Old 12-30-2012
As Rudi says, we need to know how a regular expression is going to be used to be able to provide you with a regular expression that will match anything. If there is a file containing the strings:
Code:
hexagon-bx.mydomain.com
camera01
hexagon-bx/mydomain.com

it is true that the argument "[a-z0-9\-\[bu]\.*]*$" can be given to grep as a BRE or to grep -E as an ERE to match all three lines.

But, I get the feeling that what is being requested may be a pattern match rather than a BRE or ERE. And nothing that has been suggested will cause the command:
Code:
echo expression

to print the pathnames hexagon-bx.mydomain.com, camera01, nor hexagon-bx/mydomain.com if they are pathnames of existing files in and under the current directory.

We need more context to help.
Last edited by Don Cragun; 12-30-2012 at 08:23 AM.. Reason: Mistaken sentence removed...
# 5  
Old 12-30-2012
Solved - Regex
Thanks, Scrutinizer! I was actually trying to use if expr match "$xyz" "[a-z0-9.-] and the post you gave worked! $xyz was a variable assigned to hexagon-bu.mydomain.com. The only questions I now have is

1. Does expr match use regex differently than egrep?
2. Why do we not need to escape the period or the dash in the [z-a0-9.*] I am not sure why we would not need to escape these characters as I am relatively a newbie to regex.

Thanks for everyone's assistance and input!
# 6  
Old 12-30-2012
No, expr and egrep do not use the same types of regular expression. Although some man pages aren't very clear about what form of regular expressions are used by expr match string pattern and expr string : pattern, the standards are clear:
Code:
       The  ':'  matching operator shall compare the string resulting from the
       evaluation of expr1 with the regular expression pattern resulting  from
       the  evaluation	of  expr2.  Regular  expression  syntax  shall be that
       defined in the Base Definitions volume of IEEE Std 1003.1-2001, Section
       9.3,  Basic  Regular Expressions, except that all patterns are anchored
       to the beginning of the string (that is, only sequences starting at the
       first character of a string are matched by the regular expression) and,
       therefore, it is unspecified whether '^' is a special character in that
       context.  Usually,  the	matching operator shall return a string repre-
       senting the number of characters matched ( '0'  on  failure).  Alterna-
       tively,	if the pattern contains at least one regular expression subex-
       pression "[\(...\)]" ,  the  string  corresponding  to  "\1"  shall  be
       returned.

Note that the match string pattern form is not in the standards; it is an extension supported on some (but not all) systems.

The egrep (which is obsolete) and the equivalent grep -E commands use extended regular expressions. The description of grep's regular expressions in POSIX is:
Code:
Regular expression matching shall be based on text lines. Since a <newline> separates or
terminates patterns (see the −e and −f options below), regular expressions cannot contain a
<newline>. Similarly, since patterns are matched against individual lines (excluding the
terminating <newline> characters) of the input, there is no way for a pattern to match a
<newline> found in the input.

and
Code:
       -E     Match using extended regular  expressions.  Treat  each  pattern
	      specified as an ERE, as described in the Base Definitions volume
	      of IEEE Std 1003.1-2001, Section 9.4, Extended  Regular  Expres-
	      sions.   If any entire ERE pattern matches some part of an input
	      line excluding the terminating  <newline>,  the  line  shall  be
	      matched.	A null ERE shall match every line.

You don't need to backslash-escape characters in a bracket expression in an RE. If you do, the backslash character itself becomes a member of the set of characters to be matched by a matching bracket expression or to be excluded from the list of characters to be matched by a non-matching bracket expression.
This User Gave Thanks to Don Cragun For This Post:
newbie2010
 
Login or Register to Ask a Question

Previous Thread | Next Thread

10 More Discussions You Might Find Interesting

1. Shell Programming and Scripting

Using RegEx with variable within bash if [[ ]]

I stumbled upon a problem, which I simplified to this: There is a list of numbers, stored in variable $LIST, lets use `seq 5 25` for demonstration. There is a number that should be compared against this list. For demonstration I use user input - read VALUE I am trying to compare RegEx... (2 Replies)
Discussion started by: Zorbeg
2 Replies

2. Shell Programming and Scripting

Bash regex evaluation not workin

I'm building a script that may received start and end date as parameters. I whant to make it as flexible as possible so I'm accepting epoch and date in a way that "date --date=" command may accept. In order to know if parameter provided is an epoc or a "date --date=" string I evaluate if the value... (2 Replies)
Discussion started by: lramirev
2 Replies

3. UNIX for Dummies Questions & Answers

Regex for (a|b) in bash

I am trying to find files using the following by using simple bash script: if -2014 ]]; then echo "yes";fi What I need to find are any files with date 08-**-2014 so August 2014 any files. I can use if -2014 ]]; then echo "yes";fi That works fine. How do I get files beginning with 08... (1 Reply)
Discussion started by: newbie2010
1 Replies

4. Shell Programming and Scripting

Hi im new to bash scripting I want to know what does the regex expression do ??

# check host value regex='^(||1|2|25)(\.(||1|2|25)){3}$' if ')" != "" ]; then if ]; then echo host $host not found exit 4 fi elif ]; then echo $host is an invalid host address exit 5 fi (1 Reply)
Discussion started by: kevin298
1 Replies

5. Shell Programming and Scripting

Bash regex help

I've been using the following regex below in a bash script on RHEL 5.5 using version GNU bash, version 3.2.25(1)-release I've tried using the script on RHEL 6.3 which uses GNU bash, version 4.1.2(1)-release I assume there's been alot of changes to bash since that's quite a jump in revisions.... (12 Replies)
Discussion started by: woodson2
12 Replies

6. Shell Programming and Scripting

[BASH] Allow name with spaces (regex)

Hey all, I have a very simple regular expression that I use when I want to allow only letters with spaces. (I know this regex has a lot of shortcomings, but I'm still trying to learn them) isAlpha='^*$'However, when I bring this over to BASH it doesn't allow me to enter spaces. I use the... (3 Replies)
Discussion started by: whyte_rhyno
3 Replies

7. Shell Programming and Scripting

Bash regex

Hello everybody, I'm clearly not an expert in bash scripting as I've written maybe less than 10 scripts in my life. I'm trying to strip an xml string removing every tag in it. I'm using bash substitution to do so, but apparently I missed something about what is a regex for bash ... As an... (4 Replies)
Discussion started by: kerloi
4 Replies

8. Shell Programming and Scripting

Bash string replacement - how to use regex?

Hello I have a bash script where I need to do a substring replacement like this: variable2=${variable1/foo/bar} However, I only want "foo" replaced if it is at the end of the line. However, this does not work: variable2=${variable1/foo$/bar} as you can see I'm using the $ regex for... (2 Replies)
Discussion started by: Ubuntu-UK
2 Replies

9. Shell Programming and Scripting

bash regex =~ case insensetive, possible?

It can get very annoying that bash regex =~ is case-sensetive, is there a way to set it to be case-insensetive? if ]; then echo match else echo no match fi (8 Replies)
Discussion started by: TehOne
8 Replies

10. Shell Programming and Scripting

regex test in bash

Hi I want to do a regex test and branch based on the test result, but this doesn't seems to work :confused: if \) ]] then echo success else echo failed fi (1 Reply)
Discussion started by: subin_bala
1 Replies
Login or Register to Ask a Question