I occasionally need to use regular expressions to do complex search and replace operations on LibreOffice Calc spreadsheets. It’s hard to remember all the cryptic codes, especially if you don’t use regular expressions often or are used to the syntax of a different regular expression engine. The LibreOffice help takes you to their List of Regular Expressions but it only has the more common options. The full list can be found at the ICU Regular Expressions engine documentation. I have put together nearly all of the available options in a 1-page quick reference sheet to make them easier to find. For more detailed documentation and examples, see the above two web pages. Other good references are regular-expressions.info and rexegg.com.
Please let me know if you find any errors or omissions from this sheet. The only thing I intentionally left out was alternate POSIX-like syntax such as [:alpha:] that have simpler alternatives.