There are many regex syntax, one different from the other: .Net regex, Java regex, PHP/PCRE regex, Python regex. etc.
Some sites for checking & testing regex expressions are just great - like Regex101 or RegExr [that is mentioned on InDesignSecrets grep resources], but it has different languages/syntax.
For example: The RegExr supports only JavaScript and PHP/PCRE. But the Regex Storm supports only .Net.
On what exact syntax the InDesign grep is based on? Thanks you very much for your kind help.
Edit: to be more clear - on regular-expressions.info it is called 'regular expression engine' or 'regular expression flavor'. So what is the InDesign 'regular expression flavor'?