When following this link ( https://wiki.documentfoundation.org/Documentation/How_Tos/Regular_Expressions_in_Calc ) it only points towards a link on the openoffice.org site. https://wiki.openoffice.org/wiki/Documentation/How_Tos/Regular_Expressions_in_Calc
First, we know its Wiki, so anyone with motivation can dive in to edit.
Otherwise, confirming that the compiled Help correctly points to our Document Foundation hosted Wiki, and once there the linked ICU user guide is complete.
But, the ICU guide is a little terse for our LibreOffice users looking for help with regex in Calc.
With the many LibreOffice changes to Find and Replace dialogs; as well for ICU regex usage inside Calc functions; and I assume added features to ICU regular expression (compared to the long gone OOo Regexp engine)--should we still be pointing to legacy (2009) OOo articles on Regular expressions, especially guidance that TDF does not host?
Same situation for help for regular expression in Writer, https://wiki.documentfoundation.org/Documentation/How_Tos/Regular_Expressions_in_Writer
Yes, it's wiki, so anyone can help. Ofcourse. But only an insider will know which part(s) is/are (still) relevant to LO as some of the items might have changed. And the images & text are copyrighted by openoffice.org / apache, I guess. So you might not simply do a copy/paste.
This ticket should be moved to Redmine. Although I do not want to move it, I link to a private Remdine ticket (and thus cannot tell anything about the content of that ticket)... (https://redmine.documentfoundation.org/issues/1590)