How to change the first two uppercase characters on each line to lowercase in Vim?

I have the follo开发者_运维百科wing text:

HELLO TO STACKOVERFLOW
WELCOME TO STACKOVERFLOW

I want the text to be changed as follows:

heLLO TO STACKOVERFLOW
weLCOME TO STACKOVERFLOW

How can I do that in Vim?

Highlight a visual block for the first two colums with CTRL-V

Enter gu or u to downcase the selected text, gU or U to upcase.

While @Jin provided a good answer for interactive use, here's a way to do it in scripting:

to run to every line of the buffer:

:%normal 0gu2l

or you can specify a line range where to apply the command. This will apply for lines 4 and 5:

:4,5normal 0gu2l

In normal mode:

• if startofline is set (:verb set sol? will tell you) you can use: lguCTRL-VG.
  Detail :
  • l goes to next character
  • gu is the make lowercase operator, expecting a motion
  • CTRL-V specifies that the motion is blockwise
  • G goes to first column in last line.
• if startofline is not set, then guCTRL-VGl. (l goes to next character, and . repeats the same command).

For changing to uppercase change gu with gU, for switching case ensure that tildeop is set and use ~ instead.

In addition to answers given by @Benoit, @Jin and @progo:

:%s/^../\L&\E/

see :help sub-replace-special

You can use the substitution

:%s/.*\%3c/\L&

which takes advantage of the \%c search pattern atom matching the character at a specific column on a line. Using that atom you can easily adjust the pattern to match whatever number of first characters on a line.