Bash if statement with regex

Bash if statement can be used with regular expression

if [[ "hello world" =~ "hello" ]] ## this is true
then
    echo "'hello world' has 'hello'"
fi

if [[ "hello" =~ "hello world" ]] ## this is false
then
    echo "'hello' has 'hello world'"
fi

if [[ "hello world" =~ e.* ]] ## this is true
then
    echo "'hello world' has 'e' followed by any character"
fi

Note that

 - the bracket should be double

 - use "=~", not "~="

 - use the regular expression without quotation mark

 - the lefthand-side should be a container