\ban ASCII word boundary
\d{3}a digit, repeated exactly 3 times
\ban ASCII word boundary
\s*any whitespace character, repeated any number of times
Group 1((?:(?!\b\d{3}\b)[^])*?) Non-Capturing Group(?:(?!\b\d{3}\b)[^])*? *?repeats the group contents below any number of times:
Negative Lookahead(?!\b\d{3}\b) \ban ASCII word boundary
\d{3}a digit, repeated exactly 3 times
\ban ASCII word boundary
[^]any character, including newline
\s*any whitespace character, repeated any number of times
Adónem számlaszáma: the literal text Adónem számlaszáma: (case sensitive) \S*any non-whitespace character, repeated any number of times
g modifier: global. Finds all matches instead of stopping after the first
m modifier: multiline. Causes ^ and $ to match the start and end of each line, not only the start and end of the string