#.*$ matches a single-line comment starting with a # and continuing until the end of the line. Similarly, //.*$matches a single-line comment starting with //.
If the comment must appear at the start of the line, use ^#.*$. If only whitespace is allowed between the start of the line and the comment, use ^\s*#.*$. Compiler directives or pragmas in C can be matched this way. Note that in this last example, any leading whitespace will be part of the regex match. Use capturing parentheses to separate the whitespace and the comment.
/\*.*?\*/ matches a C-style multi-line comment if you turn on the option for the dot to match newlines. The general syntax is begin.*?end. C-style comments do not allow nesting. If the "begin" part appears inside the comment, it is ignored. As soon as the "end" part if found, the comment is closed.
If your programming language allows nested comments, there is no straightforward way to match them using a regular expression, since regular expressions cannot count. Additional logic is required.