References

1. Formerly called Regex++.
2. One of fuzzy-regular-expression engines.
3. Included since version 2.13.0.
4. https://intel.github.io/hyperscan/dev-reference/getting_started.html#requirements
5. ICU4J, the Java version, does not support regular expressions.
6. C++ bindings were developed by Google and became officially part of PCRE in 2006.
7. http://www.digitalmars.com/d/2.0/phobos/std_regex.html
8. https://github.com/dotnet/corefx/blob/7116584186f8f3a886616aaf8cb5d4a982c60e27/src/System.Text.RegularExpressions/src/System/Text/RegularExpressions/Regex.cs#L2
9. https://github.com/dotnet/corefx#license
10. https://stackoverflow.com/questions/3221067/regular-expressions-in-ocaml#comment3323649_3221067
11. Non-greedy quantifiers match as few characters as possible, instead of the default as many. Note that many older, pre-POSIX engines were non-greedy and didn't have greedy quantifiers at all.
12. Shy groups, also called non-capturing groups cannot be referred to with backreferences; non-capturing groups are used to speed up matching where the group's content does not need to be accessed later.
13. Backreferences enable referring to previously matched groups in later parts of the regex and/or replacement string (where applicable). For instance, ([ab]+)\1 matches "abab" but not "abaab".
14. http://www.boost.org/doc/libs/1_47_0/libs/regex/doc/html/boost_regex/syntax/perl_syntax.html#boost_regex.syntax.perl_syntax.recursive_expressions
15. http://www.boost.org/doc/libs/1_47_0/doc/html/xpressive/user_s_guide.html#boost_xpressive.user_s_guide.grammars_and_nested_matches.embedding_a_regex_by_reference
16. FREJ have no repetitive quantifiers, but have "optional" element which behaves similar to simple "?" quantifier.
17. As of ES2018
18. Lua's only non-greedy quantifier is -, which is a non-greedy version of *. It does not have non-greedy versions of + or ?; in the former case, the non-greedy effect can be achieved by repeating the token followed by -, but in the latter case, there is no equivalent.
19. Supported by the optional regex library only. https://pypi.org/project/regex/#recursive-patterns-hg-issue-27
20. Also known as flags modifiers, modes modifiers or option letters. Example pattern: "(?i:test)".
21. Also called independent sub-expressions.
22. Similar to back references, but with names instead of indices.
23. https://www.unicode.org/reports/tr18/
24. Special feature allowing to match balanced constructs without recursion.
25. Refers to the possibility of including quantifiers in look-behinds, thus making their length unpredictable.
26. Unicode property support may be incomplete (products are continuously updated!). All will be incomplete when a new Unicode revision is released until they are updated to comply.
27. Available as of ICU55.
28. Available as of JDK7.
29. The support and range of properties is dependent on implementation.
30. "ECMA-262, 9th edition, June 2018 ECMAScript® 2018 Language Specification". https://www.ecma-international.org/ecma-262/9.0/#sec-runtime-semantics-unicodematchproperty-p. Retrieved 4 August 2020. 
31. Experimental support added in v5.29.9.
32. Supported by the optional regex library only. https://pypi.python.org/pypi/regex
33. May only be available in the regex library when used with Python versions after 3.3.
34. Means the format can be used internally without explicit conversion.
35. Partial match of the whole regular expression. For example the pattern ".*END$" will match any string partially, but only strings ending with END fully.
36. Supports Unicode 4.0 standard from 2003; latest plans for JDK7 include Unicode 6.0 (2011) support.
37. Implementation uses original UCS-2 support/features, so it only recognizes 64K chars total (vs UTF-16's 1,112,064 characters). A Microsoft developer-representative answered a bug report on this as "will not fix" in 2010.
38. Since version 8.30.
39. Tcl includes facilities to convert to and from UTF-8.
40. wxRegEx uses any system supplied POSIX library or if not available and for Unicode mode uses Henry Spencer's library.