Frequently Asked Question

What does the g flag do in :substitute, and how does the % range work?

Vim's substitute command is :s/pattern/replacement/flags. With no flags, it replaces only the first match on the current line. The g flag ("global within the line") tells it to replace every match on the line. So :s/foo/bar/ changes one foo on this line; :s/foo/bar/g changes all foos on this line.

A range in front of :s chooses which lines to operate on. With no range, just the current line. :5,10s/foo/bar/g covers lines 5 through 10. :.,$s/foo/bar/g covers the current line to the end of file. The shorthand % means "every line in the file": :%s/foo/bar/g is the standard "replace all foo with bar everywhere" command. Add the c flag (:%s/foo/bar/gc) and Vim stops on each match and asks for confirmation (y, n, a for all remaining, q to quit).

Two other flags matter often: i forces case-insensitive matching for this command (overriding the ignorecase setting), and e suppresses the "Pattern not found" error if there are no matches, useful when scripting. The pattern is a regular expression in Vim's own dialect, and the replacement supports back-references (\1, \2) and special items like \u (uppercase the next character) and & (the whole match).