APLawrence.com -  Resources for Unix and Linux Systems, Bloggers and the self-employed

Bash in-process regular expressions

© Tony Lawrence, aplawrence.com
September 2005

Bash acquired in-process regular expressions in version 3.0, but I never noticed, probably because most of the machines I'm using are Bash 2.05b. As I'm not necessarily in a position to upgrade any old box I happen to be working on, I tend to stick to the stuff that will work anywhere, which often means piping out to grep for a regular expression test.

However, should you have an environment where you can depend on this feature being present, in-process regexes obviously avoid firing up another process for grep and are much more neat to write or read.

The syntax is Perlish, using "=~". You also have access to sub-matches: $BASH_REMATCH is the string matched, ${BASH_REMATCH[1]} is the first parenthesided match, and so on. So, we can do:

if [[ "$input" =~ 'foo(.*)' ]]
        echo $BASH_REMATCH is what I wanted   
        echo but just ${BASH_REMATCH[1]}

and other more interesting tasks.

Note discussion of quotes in the comments!

Set 'nocaseglob' for case insensitivity. The return value for these is 0 for a match, 1 for no match, and (how thoughtful) 2 if the expression is syntactically incorrect, so watch out for the other side of that "if" - a non-zero return might mean you screwed up your pattern.

(OLDER)    <- More Stuff -> (NEWER)    (NEWEST)   

Printer Friendly Version

-> -> Bash in-process regular expressions


More Articles by

Find me on Google+

Tony Lawrence

Click here to add your comments
- no registration needed!

Mon Aug 4 05:18:06 2008: 4465   Taco

When trying this, I could only get it to work without the single quotes:

if [[ $input =~ foo(.*) ]]
echo $BASH_REMATCH is what I wanted
echo but just ${BASH_REMATCH[1]}

Sat Jul 18 15:57:55 2009: 6644   anonymous

me too, single or double quotes don't work... strange syntax...

Mon Mar 29 10:54:01 2010: 8294   vayerx


Any part of the pattern may be quoted to force it to be matched as a string.

Sun Aug 8 02:25:43 2010: 8878   VertigoRay


Worked for me as it was shown in the article ...
$ uname -srvo
Linux #2 SMP Sat Mar 13 00:42:43 PST 2010 GNU/Linux

Thanks for the post. Really helped me out.

Mon Sep 27 13:17:09 2010: 9012   JonathanCross


I must use unquoted regex as well.

$ if [[ sam =~ ^bob|sam$ ]];then echo match;else echo NO match;fi
$ if [[ sam2 =~ ^bob|sam$ ]];then echo match;else echo NO match;fi
NO match

$ bash --version
GNU bash, version 3.2.39(1)-release (x86_64-pc-linux-gnu)
Copyright (C) 2007 Free Software Foundation, Inc.

Mon Jun 9 14:57:12 2014: 12480   Rei


Thx !

And for invert match:
if ! [[ "$input" =~ 'foo(.*)' ]]
echo $BASH_REMATCH is what I wanted
echo but just ${BASH_REMATCH[1]}

Some !~ operator doesn't work.

Sat Feb 21 10:36:53 2015: 12613   7d42)


As others have noted, the regex pattern should not be quoted. If quotes are literally part of the match, then they should be escaped with a backslash. Quotes can be used to match a string, but in that case why bother using regex?

Ideally the example would begin as follows:
[[ $input =~ foo(.*) ]]

Note that the left side does not need to be quoted here (word splitting and pathname expansion are not performed for [[ expressions).

You used the example [[ sam =~ ^bob|sam$ ]]. This will match strings starting with bob (i.e. bobby) and/or ending with sam (i.e. rossam). You probably want this: [[ sam =~ ^(bob|sam)$ ]]

Sat Feb 21 11:41:01 2015: 12614   TonyLawrence


Well, as you can see from the comments, other people found quotes worked. I can assure you that they did when I wrote this, but today on the same machines, it does not - of course Bash has been upgraded more than once since then - it's been almost ten years!

Don't miss responses! Subscribe to Comments by RSS or by Email

Click here to add your comments

If you want a picture to show with your comment, go get a Gravatar
Kerio Samepage

Have you tried Searching this site?

Unix/Linux/Mac OS X support by phone, email or on-site: Support Rates

This is a Unix/Linux resource website. It contains technical articles about Unix, Linux and general computing related subjects, opinion, news, help files, how-to's, tutorials and more.

Contact us

Jump to Comments

Many of the products and books I review are things I purchased for my own use. Some were given to me specifically for the purpose of reviewing them. I resell or can earn commissions from the sale of some of these items. Links within these pages may be affiliate links that pay me for referring you to them. That's mostly insignificant amounts of money; whenever it is not I have made my relationship plain. I also may own stock in companies mentioned here. If you have any question, please do feel free to contact me.

I am a Kerio reseller. Articles here related to Kerio products reflect my honest opinion, but I do have an obvious interest in selling those products also.

Specific links that take you to pages that allow you to purchase the item I reviewed are very likely to pay me a commission. Many of the books I review were given to me by the publishers specifically for the purpose of writing a review. These gifts and referral fees do not affect my opinions; I often give bad reviews anyway.

We use Google third-party advertising companies to serve ads when you visit our website. These companies may use information (not including your name, address, email address, or telephone number) about your visits to this and other websites in order to provide advertisements about goods and services of interest to you. If you would like more information about this practice and to know your choices about not having this information used by these companies, click here.