str contains tabs and multiple spaces

Code:

str="hello                       world. How are    you?"

I want to check string start with hello world,
and my code is:

Code:

if [[ $str == "hello[[:blank:]]world"* ]]; then
  echo "found"
else
  echo "not found"
fi

Not work

Other solution may work is to replace all tabs and spaces with a single space. I googling and found a solution

Code:

	shopt -s extglob
	temp=$(echo "${str//+([[:blank:]])/ }")
	if [[ $temp == "hello world"* ]]; then
		echo "found"
	fi

But I don't want to replace, just check in condition. I am new to shell, sorry.