If I have the string hello and I want to replace the second and third character with _, how can i do that, given only the location of the substring, not what it actually is.
str = str.replace( /^(.)../, '$1__' );
The . matches any character except a newline.
The ^ represents the start of the string.
The () captures the character matched by the first . so it can be referenced in the replacement string by $1.
Anything that matches the regular expression is replaced by the replacement string '$1__', so the first three characters at the start of the string are matched and replaced with whatever was matched by the first . plus __.
