Could you provide some test cases?

@tsh: There's lots of zalgotext in the problem statement, but I went ahead and added a few more corner cases for you.

@tsh: Yes you may. Careful you don't normalize too soon; you might normalize a combining character that's supposed to be removed.

Does the combinator character only mean 0300-036f, or does it include other blocks?

@Naruyoko I'm using \u0300-\u036f\u0489 which is used by the site linked in this post.

Oh n̛o͢ d̑̈́ͩonͣ'̌tͤ͗̾ d̳͍̮̬̻͍̘ͭͣ͐̓̅͒͐ẽ͈̤̟͚̭͔̫̓̈́͐̔̅s̬̻̑̍ͯ̎̄́t͕̱̗̣͂̑ͤ̈̃̏r̰ͭ̇̌oͧͨ̃͛̃‌​̖̺̳y̞̟̯̖̥̎ͥ̏̅̈́̉ a͓͓̤̿͊̈̋̚̚r̨͇̺̝͗̄͘t̸͔͚͍̼̜̥̱͑̋̆͑̐̀̾͘ p̶̯̖͎̪̮̣̱͚͎͌͐̇̄̓ͪ͗͗̐ͪ̓̈́ͭ̿̑̏͘͢ḽ̵̢̭̯̦̗̬̭̳͇̖͖̺ͩ̆̾ͫ̐ͧ̈ͮ͂͒ͥͮͮ͘͢z‌​̶͍͉ͯͪͣͧ̽̐̆͒͑̆̅͗̓̒̎͜͞‌​̟̖̺̙ͅz̢̨͚͕͒̊ͦ͂ͦ͒ͦͭ̏̔‌​̪̻z͒͗͐͒ͫ̇ͭ̄̂ͤ̆͊̐ͬ͛͒̚‌​̥̫͇͚̝̾͡z̶̃ͣ̐̅͐̿́̚͏̷̳‌​͍̹̗z̸̡̡ͮͪ́̃̈̍̽͂̑̇ͭ͜͠‌​̖͚̤̣̲̥̣̼zͪ͌ͣ̈́ͪͪ̒͒̄ͩ̉‌​̡̓ͨͨ̄͏͝҉͍̟̯̳̰̥̟̫ͅz͑̂‌​̵̛͊̇̚҉̧̹͕͍̼̕z̐ͫ̈ͯͫ͆͋‌​̀͏҉̙̼͙̙̙z̷̛̛̯͐̽͛ͦ͐̇̑‌​̞͎̜͓̻̹̹̼̩̫̝͙̝z̸ͯͦ̆̅ͯ‌​͓̻̻̥͙͈̫̘͈̕z̎̒͑ͧ̍ͮ̈́͒͡‌​̸͔̳̩̯̹͚̱̣͎̘͉̲!ͪͪ͋̅̍̑‌​̸̷̨̪̣̤̜̗͎̞̘͍̞̜̥̳̀͋́͢‌​̝͙͓͕̤̲!̸̨̡͎͐͑̇͐͒̎͂̀͜‌​̣͖̩̲̙̙͖̖̙̲!̔͌̏̓̈ͮ͗ͭ͂‌​̷̺̣̘̗̞͔͍͙́̀ͅ!ͣͮͪ̈́̔ͫ̚‌​̵̶̛͔̗̘̺̋͐̀͊͊̔͢!̊̏̀̽ͭ‌​̸̛͉̻̬̤̖͓̥̺̻͔ͮ͊̓̑̄̉̑́‌​͖͚!̴͕̺͔̿͒ͯ̇ͣ͋̀̐͒̿̓ͤ͟

It doesn't really matter since your rules are clearly specified, but English also has a few borrowed words with accents you didn't mentioned: café, résumé, ...

Nitpick: are you really “undoing” the operation if you don’t get the original text? A more difficult challenge would be to remove the permission to delete instead of fix the characters.

Should words with grave accents (e.g. "blessèd") keep or lose the accent? (Grave accent and diaeresis marks being the only 2 native diacritics in Modern English)

@WGrosleau: the zalgo generator only adds combining characters.

@Chronocidal: I'm not going to change that rule now but yeah that should have been how it worked.

I love how my browser asks me if I'd like to translate this page... and then proceeds to rewrite the text in Vietnamese.

Well, that question from @Chris Porter on the famous Stack Overflow answer is finally answered. It is possible to parse that answer with a regex!

@TejasKale, I tried to leave a comment earlier but the answer is locked.

@Nat That's epic! I had to turn off mathjax temporarily to be able to upvote your comment ...

@Nat How do you do that?

@facepalm42 \$\text{Right-click}\$ \$\to \texttt{Math Settings}\$ \$\to \texttt{Math Renderer}\$ \$\to \texttt{Plain Source} .\$ (Plus you want to put \$\mathrm{\TeX}\$ tags around it, which on this site, are \$. For example, \$x\$ renders as \$x .\$)

@Nat \texttt{Why isn't it working for me?} I set the math renederer back to HTML/CSS and it's still showing as plain text.

If anyone wants to read more about Cthulhuian madness - erm, I mean, \$\mathrm{\TeX}\$ - I wrote a short intro here, and there's also a guide at SE.Math. On SE.CodeGolf, the delimiters for in-line \$\mathrm{\TeX}\$ are \$ instead of the normal $, but it's otherwise the same.

My Chrome browser insists that this page is in Vietnamese and keeps trying to automatically translate it, which renders 60% of the question unreadable. Took me 10 minutes before I realized that the poster wasn't just cleverly making half of the question unreadable unless we already had a partially working unZalgo.