Did I just find a bug in Notepad++’s lookbehind implementation

notepadregex

I am using Notepad++ v7.3.3, and am testing the following Regex:

Source text

aa

Pattern

(?<!a)a

This is returning two matches. The first a, and the second a.
When testing this expression on regex101 it only returns one match (the first a).

Why is Notepad++ also matching the second a? Could this be a bug?

Best Answer

It's not a bug... Notepad++ "search" when you have DOWN selected, does not recognize any characters BEFORE the cursor. When you selected the first a, your cursor moved. So once your cursor is passed the first a, it then matches the remaining a. The same could be said about searching "UP", it will ignore any character from the cursor to the end of the document.

Your regex hasn't failed, it's just you have to remember and understand the nature of the text editor in general.

Regex 101 didn't perform a "partial" string lookup like Notepad++ did. However you can correct this by using anchors in your regex. You could Anchor this regex to the start of the line with ^. Then you wont get a match on your second a.

^(?<!a)a

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I also couldn't find a way to select and copy every marked match. I'm a fairly novice NP++ user though, so maybe someone else knows how to do this.

However, you could instead search for everything you don't want to copy and replace it with nothing. This will leave you with only what you want, so you can copy it or use it as you like. Then just be sure to undo, save the result as a new file, or don't save at all in order to keep your original data intact.

Based on your comments, you're trying to grab hashtags from some text. You can use the following regex pattern to match everything but hashtags:

(?<!#)\b[^#]+

Replace all with nothing or a space.

Explanation of pattern:

\b[^#]+ maximally matches text, starting from a word boundary, that does not include a pound sign.
(?<!#) is a negative look-behind for the pound sign. This prevents matching any text immediately preceded by a pound sign.

This should leave nothing but hashtags behind. [^#]+ seems to be matching newline characters as well, so this will leave all your hashtags on a single line.

Best Answer

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