I have tried to create a regular expression that would match if the input text has max 3 lines of text, at most 10 characters per line, and characters are all uppercase.
So this string should match: "AA\n\nAA"
but this shouldn't
"A12c"
I thought this would work: (I enabled multiline in Pattern)
(^[A-Z]{0,10}$){0,3}
but it doesn't, it only matches if the text is jut one-liner.
I cannot understand what is wrong with the expression - isn't the {0,3} quantifier applied correclty?
You forgot to match the line terminator:
(^[A-Z]{0,10}$\r?\n?){0,3}
should work, assuming that the option for ^ and $ to match start/end-of-line and not start/end-of-string is set.
If you need the regex to fail if there are more than 3 lines in your string, you can force the regex engine to match the entire string or not at all by surrounding it with \A and \z anchors:
\A(^[A-Z]{0,10}$\r?\n?){0,3}\z
However, not all regex flavors support these start-of-string/end-of-string anchors.