How to find a specific portion of a string using a regular expression in bash

Question

I would like to find a specific portion of a string using a regular expression in bash. Additionally, I would like to store that portion in another bash variable. By searching in the web, I could find one solution:

#!/bin/bash

string="aaa.ddd.config.uuu"
result=`expr match "$string" '\(.*\)\.config.*'`
echo $result

In the above case, I would like to find out the portion before "\.config" and want to store in another variable named result. Is the above one a good and efficient approach? I would like to know what would be a recommended way in such cases.

F. Hauri - Give Up GitHub · Accepted Answer · 2019-11-20 14:01:09Z

2

Avoid bash regex when not needed

Use:

result=${string%.config*}

Not only quicker, but more POSIX compliant.

Tests:

string="aaa.ddd.config.uuu"

time for ((i=30000;i--;)){ [[ $string =~ (.*)\.config ]] && result=${BASH_REMATCH[1]} ;}

real    0m1.331s
user    0m1.313s
sys     0m0.000s

echo $result 
aaa.ddd

Ok

time for ((i=30000;i--;)){ result=${string%.config*} ;}

real    0m0.226s
user    0m0.224s
sys     0m0.000s

echo $result 
aaa.ddd

For this, bash regex will take 5.8x more resources than simple Parameter Expansion!

answered Nov 20, 2019 at 14:01

F. Hauri - Give Up GitHub

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2 Comments

Walter A Over a year ago

Parameter expansion is excellent. The BASH_REMATCH is much better than an external solution like sed 's/[.]config.*//' <<< "${string}".

F. Hauri - Give Up GitHub Over a year ago

@WalterA Depending on size of files to work on! Using dedicated tasks to do specialized function (like sed) permit parallelization, then become much more quick! Have a look at best way to divide in bash for a sample of dedicated parallel task!

Maxim Egorushkin · Accepted Answer · 2019-11-20 13:45:11Z

2

You can use bash built-in regular expressions and the array variable BASH_REMATCH that captures the results of a regular expression match:

$ string="aaa.ddd.config.uuu"
$ [[ $string =~ (.*)\.config ]] && result=${BASH_REMATCH[1]} 
$ echo $result
aaa.ddd

answered Nov 20, 2019 at 13:45

Maxim Egorushkin

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