Your comments

The "Whole Word" search seems very unreliable when leading whitespace is involved. Make a document reading:

a a
a a

(Two lines, A space A on each) Menu > Replace...

Enable "Whole Word" and enter " a" (a space followed by an A) and set the replacement string to "b". Click find and see that the first " a" is highlighted, but clicking replace has no effect. What's more, a document with only " a" will not have " a" found in it when Whole Word is enabled. The status bar shows:

Unable to find \b a\b

Indeed, \b does not seem to match ^ in regex