Bash – regex

if [[ "$string" =~ ‹pattern› ]]; then
    echo "Match found!"
else
    echo "No match."
fi

pattern="[a-z]+"

if [[ "$string" =~ $pattern ]]; then
    echo "Match found!"
else
    echo "No match."
fi

filename="document.txt"

if [[ "$filename" =~ \.txt$ ]]; then
    echo "Match found!"
else
    echo "No match."
fi