check if command starts with a letter unix code example

Example 1: 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

Example 2: check if word at end of string regex bash

Just appreciate next solution example for your code implementation:
S=finding
if[[ "$S" == *ing ]]; then ...//some code
More generally:
if[[ "string_to_check" == regex_patten ]]; then ...//some code