Move all text between two "\t" in Notepad++

I have more than a million lines of text in this format:

AAAA BBBBBBBBBBBBBBB CCCC

Separated by \t

I want to have it in a format

AAAA_CCCC BBBBBBBBBBBBBBB

But I cannot seem to figure out how to do it using regular expressions in Notepad++

You may try the following find and replace, in regex mode:

Find:    ^(\S+)\t(\S+)\t(\S+)$
Replace: $1_$3 $2

Here is a demo.

If the separator is a tab, you can use

^[^\r\n\t]+\K\t([^\r\n\t]+)\t([^\r\n\t]+)$

The pattern matches:

• ^ Start of string
• [^\r\n\t]+ Match 1+ chars other than a tab or newline
• \K\t Forget what is matches so far using \K and match a tab
• ([^\r\n\t]+) Capture group 1, match any 1+ chars other than a newline or tab
• \t Match a tab
• ([^\r\n\t]) Capture group 2, match 1 char other than a newline or tab
• $ end of string

In the replacement use the 2 capture groups with an underscore in between.

_$2 $1

See a regex demo.

The result of the replacement:

AAAA_CCCC   BBBBBBBBBBBBBBB