vim tutorial – using the s command to replace text on the fly

The vim text editor comes with a powerful :s (substitution) command that is more versatile and expressive than the search and replace functionality found in GUI based text editors.

The general form of the command is:
:[g][address]s/search-string/replace-string[/option]

The address specifies which lines vim will search. If none is provided, it will default to the current line only. You can enter in a single line number to search, or specify an inclusive range by entering in the lower and upper bounds separated by a comma. For example: an address of 1,10 is lines one through ten inclusive.

You can also provide a string value for the address by enclosing it with forward slashes. vim will operate on the next line that matches this string. If the address string is preceded by “g”, vim will search all lines that match this string. /hello/ matches the next line that contains hello, whereas g/hello matches every line that contains hello.

The search-string is a regular expression and the replace-string can reference the matched string by using an ampersand (&).

[option] allows even more fine grained control over the substitution. One of the more common options used is “g”, not to be confused with the “g” that precedes address. Option “g”, which appears at the end of the command, replaces every occurrence of the search-string on the line. Normally, the substitute command only matches on the first occurrence and then stops.

s/ten/10/g

run on the following line:
ten + ten = 20

results in:

10 + 10 = 20

as opposed to:

10 + ten = 20

without the global option.

Given all this versatility, the :s command comes in quite handy. Consider the following scenario. There is a comma delimited file that is missing trailing commas on some lines and not others. In order to normalize the text file so that all lines ended with a comma, you could run:

1,$s/[^,]$/&,

The address range 1,$ spans the entire file ($ in the address means the last line in the file). The search-string “[^,]$” is a regular expression that matches every line that ends with any character except comma ($ in a regex indicates end of the line). The replace-string has an &, which refers to the trailing character matched in the search-string. By setting the replace-string to “&,” we are telling VIM to take the last character on every line that is not a comma and add a comma to it.

[^,]$ won’t match on blank new lines because [^,] expects at least one character to be on the line. To get around this problem, you would normally use negative look behinds, however the VIM regex does not seem to support them. The easiest way around this is to use a second replace command for newlines:
1,$s[^\n$]/,

This tells it to only add a comma to any line that only contains a newline (^ in a regex indicates start of line).

This is just one example of course. By coming up with the right regex in the search-string, you can automate all sorts of normally tedious tasks with succinct commands. The best part is, unlike those cumbersome GUI based editors that often require the use of a pesky mouse, your hands never have to leave the keyboard! For even more control and flexibility, you could use sed, but :s can handle most day to day tasks quite easily.