FAQ: Where to find REGular EXpressions (RegEx) documentation ?

guy038

IMPORTANT : updated on February 28, 2025 : I modified the last part of this post, after the REMARK point. I deleted the part relative to the old N++ v.6.9.0 release and replaced it with the new release of the N++ Columns++ plugin of @Coises.

Hello and welcome to the FAQ Desk. You have likely been directed here because you asked about N++ regex flavor, [ N++ ] regex documentation or N++ regex enhancements.

PRELIMINARY note : As recalled recently by @peterjones, our forum is not a professional regex-writing forum. We are users of Notepad++, who want to help others learn more about NPP (and learn more ourselves, in the process). Since Notepad++ makes use of regular expressions, we will often provide help with regex questions.

Our present regex engine is implemented since Ascii and Unicode version 6.0 of Notepad++ and improved till the v6.4.2 version, by, both, Dave BrotherStone and François-R Boyer. Previously, It used the Regex library, of the Boost C++ libraries v1.55.0, similar to the PERL Regular Common Expressions v5.8.

Starting with Notepad++ v7.7, it currently used the Boost C++ regex library v1.70.0. Since this release, the Backtracking Control Verbs are supported. Refer to the link, below, for information on this new feature :

https://community.notepad-plus-plus.org/post/55464

Finally, since Notepad++ v8.2, the current Boost C++ regex library is the release v1.78.0.

For noob people, about regular expressions concept and syntax, begin with that article, elaborated by Peter Jones and part of the Notepad++ Documentation :

https://npp-user-manual.org/docs/searching/#regular-expressions

Note that the old link of the outdated Notepad++ Wiki ( http://docs.notepad-plus-plus.org/index.php/Regular_Expressions ) as well as the temporary link, on GitHub ( http://notepad-plus-plus.github.io/npp-usermanual/content/searching/ ) are, from now on, obsolete

In addition, whatever its level, anyone will find good documentation, about the regex library; of the Boost C++ libraries v1.70.0, from the two addresses below :

http://www.boost.org/doc/libs/1_78_0/libs/regex/doc/html/boost_regex/syntax/perl_syntax.html

http://www.boost.org/doc/libs/1_78_0/libs/regex/doc/html/boost_regex/format/boost_format_syntax.html

The FIRST link explains the syntax, of regular expressions, in the SEARCH part.
The SECOND link explains the syntax, of regular expressions, in the REPLACEMENT part.

And, possibly, take a glance at the Standards Conformance :

https://www.boost.org/doc/libs/1_78_0/libs/regex/doc/html/boost_regex/background/standards.html

You may, also, look for valuable information, on this non-exhaustive list of sites, below :

General Information :

https://en.wikipedia.org/wiki/Regular_expression

Summary of Regex Syntax ( Cheat Sheet and Basic Features ) :

https://www.regular-expressions.info/quickstart.html

http://www.rexegg.com/regex-quickstart.html

https://regex101.com/ — Quick Reference panel ( Thanks to Jim Dailey )

https://medium.com/factory-mind/regex-tutorial-a-simple-cheatsheet-by-examples-649dc1c3f285

https://ryanstutorials.net/regular-expressions-tutorial/ — Excellent introduction

https://www.geeksforgeeks.org/write-regular-expressions/

https://docs.python.org/2/howto/regex.html

https://docs.microsoft.com/en-us/dotnet/standard/base-types/regular-expression-language-quick-reference

https://www.debuggex.com/cheatsheet/regex/pcre

http://regexstorm.net/reference

Full Documentation and Advanced Information :

http://www.regular-expressions.info ( the REFERENCE ! )

http://www.rexegg.com

https://gammon.com.au/pcre/pcrepattern.html

http://www.pcre.org/pcre.txt

http://perldoc.perl.org/perlre.html

https://stackoverflow.com/questions/22937618/reference-what-does-this-regex-mean

Regex Testers :

https://www.regexbuddy.com — All flavors ( the REFERENCE ! )

https://regex101.com/ — PCRE , Python, JavaScript and Golang flavors

https://regexone.com/ — PCRE flavor

https://www.debuggex.com/ — JavaScript, Python and PCRE flavors ( Visual regex representation )

https://www.regextester.com/ — JavaScript and PCRE flavors

https://www.regexpal.com/ — JavaScript and PCRE flavors

https://www.freeformatter.com/regex-tester.html — JavaScript flavor

https://regexr.com/ — .NET flavor

Be aware that, as any documentation, it may contain some errors ! Anyway, if you’ve detected one, that’s good news : you’re improving ;-))

I highly recommend reading the contents of the third edition of the book Mastering Regular Expressions ( Understand your Data and Be More Productive ) by Jeffrey E. F. Friedl ( August 2006 ). If you find it of interest, just download this PDF file, of 542 pages.

Refer for an English version ( ISBN 0-596-52812-4 ) from the link, below :

https://github.com/pengbo-learn/books/blob/master/Mastering Regular Expressions 3rd (Friedl-2006).pdf

And click, on your right, on the button Download raw file in order to download the PDF file.

Quotations :

Some people, when confronted with a problem, think “I know, I’ll use regular expressions.” Now, they have two problems! — Jamie Zawinski

And, of course, one claimed precedence. One said: “I needed love, I got married. Then, I had two problems”! — Anonymous

REMARK :

After some practice, you will probably find out some unexpected results with our present N++ regex engine :

It does not completely handle the characters over the BMP, due to the present management of UTF-16 code units.
And thus, you can only access the characters over the BMP with the surrogate pairs ( \x{D8..}\x{D...} ) mechanism, instead of the classical \x{#####} or \x{######} notation.
The management of accentuated characters and the NUL character, in replacement, does not work.

Now, starting with the N++ release v8.7.8, you’ll be able to use an enhanced version of our Boost regex engine ( Big thanks to @Coises !! ) which corrects some bugs and adds a lot of regex improvements.

Refer to the following links :

https://community.notepad-plus-plus.org/topic/26659/columns-version-1-2-better-unicode-search

https://github.com/Coises/ColumnsPlusPlus/releases/tag/v1.2

It is important to emphasize that the Find and Replace option of the Columns++ plugin does not replace the native N++ Find and Replace dialog, in many ways :

This plugin cannot search ( and replace ) in all opened files or the current N++ session
This plugin cannot search ( and replace ) in all files, with specific(s) extension(s) of a tree structure

However, in return :

This plugin allows search ( and replace ) in one or more rectangular selections and the ^ and $ assertions match at beginning and end of each selection row !
This plugin allows mathemathical formulas in replacements !

Here are two links to the on line help about the Search and Regular Expressions sections to clarify when Columns++ might be useful.

However, take a tour of Columns++'s powerful features !!

Now, to give it a try, install the ColumnsPlusPlus plugin from the N++ option Plugins > Plugins Admin... , and, then, select the N++ option Plugins > Columns++ > Search...

Regarding the whole documentation, run the Plugins > Columns++ > Help/About... > —> Help option

The main improvements of this enhanced BOOST regex engine are :

Search and replacement are both performed in true 32 bits code-points.
So, it can handle ALL the Universal Character Names ( UCN) of the UCS Transformation Format , from \x{0} to \x{7FFFFFFF}, particularly, all those of code-points over \x{FFFF}, which are outside the BMP ( Basic Multilingual Plane ).
Both, search and replace strings can contain embedded NUL characters and/or Escape sequences for NUL characters ( \x{0000} ).
Backward assertions, as \A, are correctly supported ( OK with N++ since v7.9.1 ).
Look-behinds are correctly handled, when searching or replacing, even those overlapping with the end of the previous match ( OK with N++ since v7.9.1 ).
The management of accentuated characters, when case replacement, works as expected.
All POSIX characters classes as well as some single letter classes and the [[unicode:]] class work correctly. Refer to the link :

https://www.boost.org/doc/libs/1_87_0/libs/regex/doc/html/boost_regex/syntax/character_classes/std_char_classes.html

All Unicode character classes work correctly, with the exception of the \p{Cs} and \p{Surrogate} classes. Refer to the link :

https://www.boost.org/doc/libs/1_87_0/libs/regex/doc/html/boost_regex/syntax/character_classes/optional_char_class_names.html

Important note :

The classes of characters, sensible to the case, will always return case-sensitive results, even if the Match case option is not checked or a (?i) modifier is used !

Some escape sequences have been added :
- \o for ASCII code-points, between 0 and 127.
- \y or [[defined:]] for defined characters, except for the three Unassigned, Invalid and Private use characters.
- \i or [[invalid:]] for INVALID UTF-8 bytes or sequences.
All Control and Non-printing character abbreviations can be found, independently, with the Symbolic character name [[.XXX.]] ( e.g. [[.NBSP.]] would find all the Non Breaking Space characters ).
The \X feature has been improved and reliably matches a grapheme cluster, so an usual single character, regardless of how many Unicode code-points are necessary to compose this character ! ( It’s, for example, the case of a base letter with subsequent accentuated characters or of some Emoji sequences in order to build combined emojis )
The \K feature ALSO works in a step-by-step replacement with the use of the Replace button

Finally, Columns++ plugin shows a process dialog when the estimated time, for multiple Count, Select or Replace actions, exceeds two seconds !

REMARK : Although the improvements made to Columns++ plugin mainly concern Unicode handling, this plugin works perfectly well with ANSI files, too !

Now…, just enjoy any complicated regex in search/replacement !!

MAPJe71

Additional Regex Tester tool (offline): RegEx Tester