Regex to trim whitespace, collapse runs and drop blank lines

For a plain whole string trim there is no argument: the built in method is shorter, faster and cannot be got wrong. The regex version, /^\s+|\s+$/g, is worth knowing because it is the shape you extend when the requirement grows, and because seeing the alternation makes it obvious why the g flag is required and why both halves are anchored.

Collapsing runs is where a pattern genuinely helps. Replacing every /\s+/ with a single space flattens tabs, repeated spaces and line breaks into one separator, which is exactly right for a search index and exactly wrong for a document, because paragraph structure disappears with it. When the line structure has to survive, collapse [ \t]+ instead and leave the terminators alone.

Per line trimming is the case that actually needs the m flag. Anchoring with ^ and $ under m applies the trim at every line boundary, and using [ \t] rather than \s is what stops the pattern eating the line terminators themselves and joining the lines together.

Removing blank lines is easier expressed as a replacement between terminators than as an anchored pattern. Replacing /\n\s*\n/g with a single newline collapses any run of empty or whitespace only lines down to one break, and it leaves a single blank line at the start or end of the text alone, which is usually what you want.