bash - grep regex not returning result when regex stored in shellscript variable -

let's helloworld.txt contains

function( '100');

now want search occurence of '100' after ( may have space in between. now, unix command:

case-i:

grep "[(][ ]*'100'" helloworld.txt

i correct match found in helloworld.txt.

but when write simple shellscript:

case-ii:

key="100" key="\"[(][ ]*'$key'\""  echo $key grep $key helloworld.txt 

it correctly prints: "[(][ ]*'100'"

but, doesn't return match found in helloworld.txt

i think you're overcomplicating things bit. original command can written this:

grep "( *'100'" helloworld.txt 

and if want number 100 come variable, use this:

grep "( *'$key'" helloworld.txt 

if want store whole regular expression in variable, be:

regex="( *'$key'" grep "$regex" helloworld.txt 

note syntactic quotes (like ones around string literal, or variable) not interchangeable quotes inside strings. mean following 2 things not equivalent:

regex="( *'$key'" # grep "$regex"  regex="\"( *'$key'\"" # bad grep $regex 

---

below test showing of options working:

$ cat file function( '100'); $ grep "( *'100'" file function( '100'); $ key=100 $ grep "( *'$key'" file function( '100'); $ regex="( *'$key'" $ grep "$regex" file function( '100');