regex expression to check if a string starts with a letter bash code example

Example: bash string starts with

[[ $a == z* ]]   # True if $a starts with a "z" (wildcard matching).
[[ $a == "z*" ]] # True if $a is equal to z* (literal matching).

if [[ "$HOST" =~ ^user.* ]]; then
    echo "yes"
fi

if [[ "$HOST" =~ ^user.*|^host1 ]]; then
    echo "yes"
fi