Notepad++

To cchris: the "1 per line" box was never checked. But thanks for the suggestion anyway.

To Fool4UAnyway:
Hmmm... interesting. I did a bit of experimenting and confirmed exactly what you are saying about the selection changing. That's indeed why only one column of spaces got replaced. I don't know if that was the intended functionality or not, but it seems obvious to me that text which is selected when an operation is initiated should remain selected while that operation is taking place. The behaviour you pointed out (selected text becoming invisibly unselected) seems very counter-intuitive to me.

I've tried working with Notepad++'s "regular expressions" in the past and found them almost impossible to use. Things that I have tried to do with regexes that I *know* work in Perl or Awk do not work in Notepad++, and leave me trying repeatedly to tweak spaces or periods or other characters to trick Notepad++ into doing what I want it to do. I finally gave it up as a lost cause several months ago, and I do not even attempt to use them any more.

Selecting just the column of spaces and using Alt-C as you suggest did exactly what I needed. This is probably the solution I should have used from the start, except that I was trying to "kill two birds with one stone" and replace both columns of spaces with a single operation. In hindsight, it would have been quicker to do it this way... :P

Thank you very, VERY much for your help! Not only did you tell me what I needed to know, but you gave me a glimpse into the internal workings of Notepad++ as well.