Hi, I’m totally new to =REGEX so forgive me this noob question:
If in C2 I use formula: =REGEX(A2,B2) it returns: 2021-08-27
If in C3 I use formula: =REGEX(A3,([^","]+)) it returns: Err:507
In below example: what to type in the =REGEX formula in Expression part instead of cell reference?
Thanks
You need to put the condition inside double quotes, like a text.
Still messing something up 
Hallo
the quotes inside the regex doesnt make sense, neither in B2 nor literally!
=REGEX(A5;"[^,]+")
… but just FYI, if you need double quotes inside a string literal in Calc in the future: since double quote is a special character (starting / ending a string literal), its use inside a string requires escaping it - by doubling.
E.g., if the double quotes were needed in the regex (they don’t, as @karolus mentioned), then the proper formula would be
=REGEX(A5;"[^"",""]+")Nitpick: even if in another case a double quote would be needed in a (negated) set, listing it twice is superfluous.
SCNR