How to use regex in Linux Bash

Regex or regular expression uses a pattern that is compared to a string and will check if the pattern exists or the string matches the pattern.

In Bash script a simple if then else and a regex pattern can be used to check if the pattern matches a set of input on a text file.

Bash script below is a simple example on how to use regex in Bash using if then else.

Script below uses this sample file "number_file.txt" with the sample text or contents shown below.

a12567

b23567

c34568

12w678

23908

35107

23469

x1234

y3432

12094x

123y8

====

#!/bin/sh

patx="^[0-9]+$"

while IFS= read -r line; do

    echo "Text read from file: $line"

    if [[ $line =~ $patx  ]]

               then

                 echo "Hey this one is alpahumeric --> $line."

     else

                 echo "Hey this one is a group of numbers --> $line."

     fi

done < number_file.txt

===

Above Bash script will check the number_file.txt,  each output line will be checked if it is  numbers only or has numbers and letters on it.

The operator used in Bash if then else to use regex is “=~” it’s the equal sign and the tilde character.

The script will read the file line by line, and compare the line to the regex pattern.

Regex pattern use: patx="^[0-9]+$"

Bash script below is another implementation but uses the built-in type.

===

#!/bin/sh

while IFS= read -r line; do

    echo "Text read from file: $line"

    if [[ $line =~ [^[:digit:]]  ]]

               then                        

                 echo "Hey this is alphanumeric --> $line."

     else

                 echo "Hey this is a group of numbers --> $line."

     fi

done < number_file.txt

===

Regex pattern use: [^[:digit:]]

:digit: built-in type in Bash.

Sample output of the Bash script:

That’s just a simple illustration on how to use regex in Bash script, this will be useful if the input follows a pattern or the requirements is quite simple such as to check if the string has letters or just numbers.

Till next time. Stay safe! and keep things up! 

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