regex constructor for match firct three characters code example

Example 1: regex for first three characters

^.{0,3}
.{0,3}$

Example 2: regex for strings with specific letters javascript

let re = /ab+c/;

Example 3: regex match two characters

echo %H#E%L#L%O! | sed 's/[#%]//g'	#replaces every appearance of any of
two characters # and %