Find if a string starts with a specific pattern using regex in bash script

Question

I'm trying to use regext to detect if a string starts with a specific pattern or not but it does not work with me:

   #!/bin/bash
   line="{{ - hello dear - }}"
   if [[ "${line}" =~ ^\{\{\s*-\s*hello\s*.*\}\} ]]; then
        echo "got it "
   fi

In this example, I expect the if condition to detect that the line variable has a string that starts with "{{ - hello" and ends with "}}" However, it does not do so as the echo message is not printed!

bash version is: GNU bash, version 5.0.17(1)-release (x86_64-pc-linux-gnu) — mnabil
– mnabil, Commented Jan 6, 2022 at 18:43
Please add your desired output (no description, no images, no links) for that sample input to your question (no comment). — Cyrus
– Cyrus, Commented Jan 6, 2022 at 18:52

konsolebox · Accepted Answer · 2022-01-06 19:17:21Z

2

You can store the regex in a variable to make it work. It was a workaround in 3.2. Not sure why it's needed again in newer versions.

#!/bin/bash

line="{{ - hello dear - }}"
regex='^\{\{\s*-\s*hello\s*.*\}\}'

if [[ "${line}" =~ $regex ]]; then
     echo "got it "
fi

Also consider using extended pattern matching with == instead. I believe it has a weaker "engine" but it's more readable sometimes.

shopt -s extglob
...
[[ $line == "{{ - hello dear - }}"* ]] && echo "Got it."

(Actually that's not even an extended pattern yet.)

edited Jan 6, 2022 at 19:17

answered Jan 6, 2022 at 19:11

konsolebox

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3 Comments

Fravadona Over a year ago

Warning! using \s only works on Linux (tested on macOS, FreeBSD and Solaris)

konsolebox Over a year ago

@Fravadona What version of Bash do you use? I just tested it again with 3.2, 3.2.46, 4.0, 4.4 and latest and it works.

konsolebox Over a year ago

I see you changed your comment. Yes that might be the case for non-Linux. Regex is most likely parsed by a different engine in others.

anubhava · Accepted Answer · 2022-01-06 19:23:03Z

1

There is no real need to use regex here. You can just use glob matching in bash like this:

line="{{ - hello dear - }}"

[[ $line == '{{ - hello'*'}}' ]] && echo "got it "

got it

answered Jan 6, 2022 at 19:23

anubhava

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