Sure, but that's up to whoever is writing the regular expression.

The standard '.' (match any character) in a regexp matches an Unicode code point, not a grapheme cluster. To match a grapheme cluster, you have to use "\X", which is not universally supported.

For example, in Python 3, the builtin module module "re" doesn't support "\X", you have to install the "regex" module for it:

    # text is 'e' followed by U+0301 (combining acute accent):
    text = b'e\xcc\x81'.decode('utf8') 
    print(f'text: "{text}"')

    import re
    print(re.match('^(.)(.)$', text).groups())   # prints "('e', '´')"

    import regex  # must be installed
    print(regex.match('^(\X)$', text).groups())  # prints "('é',)"

At the end of this comment there is what I believe is a single grapheme cluster. On disk this single "letter" occupies 73 bytes. Surprisingly large number of tools and editors know how to work with things like these and render them at least somehow

I think I once created one that was about a kilobyte. Is there an upper limit?

I created it using this page https://glitchtextgenerator.com/

The 73 byte X:

x̧̡̬̘͓̖̲̻̻̲̠̪̻͓͙̜̂̓̊̔̀̀͗̑̀̅̀̂̚͘̕̚͘͢͜͠