If it's really that simple, you could just use tr and stop worrying about regex special characters, since tr doesn't have regular expressions at all, just ranges and a few character classes.

The odd '"'"' bits are to get single-quote characters inside a string that's otherwise single-quotes, since you can't escape anything in single-quotes. The colors show what's actually happening -- the single quotes end, double quotes containing a single quote begin, then it goes back to single quotes.

Code:

$ echo 'Hello ,.@/#:()'"'"'*%$+?_="!; Goodbye' | tr '[,.@/#:()'"'"'*%$+?_="!;]' '_'

Hello ___________________ Goodbye

$ echo 'Hello ,.@/#:()'"'"'*%$+?_="!; Goodbye' | tr -s '[,.@/#:()'"'"'*%$+?_="!;]' '_'

Hello _ Goodbye

$

You'd need a double-backslash \\ inside that range to get a backslash, and would need to escape [ ] - characters inside the range, but that's pretty much it for specials.