JavaScript: how to use a regular expression to remove blank lines from a string?

Your pattern seems alright, you just need to include the multiline modifier m, so that ^ and $ match line beginnings and endings as well:

/^\s*\n/gm

Without the m, the anchors only match string-beginnings and endings.

Note that you miss out on UNIX-style line endings (only \r). This would help in that case:

/^\s*[\r\n]/gm

Also note that (in both cases) you don't need to match the optional \r in front of the \n explicitly, because that is taken care of by \s*.

As Dex pointed out in a comment, this will fail to clear the last line if it consists only of spaces (and there is no newline after it). A way to fix that would be to make the actual newline optional but include an end-of-line anchor before it. In this case you do have to match the line ending properly though:

/^\s*$(?:\r\n?|\n)/gm

I believe this will work

searchText.replace(/(^[ \t]*\n)/gm, "")

This should do the trick i think:

var el = document.getElementsByName("nameOfTextBox")[0];
el.value.replace(/(\r\n|\n|\r)/gm, "");

EDIT: Removes three types of line breaks.

Here's another solution:

string = string.replace(/^(?=\n)$|^\s*|\s*$|\n\n+/gm, "")

It appears to handle both blank lines and space-only lines.