6. Global Replacement

Sometimes, halfway through a document or at the end of a draft, you may recognize inconsistencies in the way that you refer to certain things. Or, in a manual, some product whose name appears throughout your file is suddenly renamed (marketing!). Often enough it happens that you have to go back and change what you've already written, and you need to make the changes in several places.

The way to make these changes is with a powerful change command called global replacement. With one command you can automatically replace a word (or a string of characters) wherever it occurs in the file.

In a global replacement, the ex editor checks each line of a file for a given pattern of characters. On all lines where the pattern is found, ex replaces the pattern with a new string of characters. For right now, we'll treat the search pattern as if it were a simple string; later in the chapter we'll look at the powerful pattern-matching language known as regular expressions.

Global replacement really uses two ex commands: :g (global) and :s (substitute). Since the syntax of global replacement commands can get fairly complex, let's look at it in stages.

The substitute command has the syntax:

:s/old/new/

This changes the first occurrence of the pattern old to new on the current line. The / (slash) is the delimiter between the various parts of the command. (The slash is optional when it is the last character on the line.) A substitute command with the syntax:

:s/old/new/g

changes every occurrence of old to new on the current line, not just the first occurrence. The :s command allows options following the substitution string. The g option in the syntax above stands for global. (The g option affects each pattern on a line; don't confuse it with the :g command, which affects each line of a file.)

By prefixing the :s command with an address, you can extend its range to more than one line. For example:

:50,100s/old/new/g

will change every occurrence of old to new from line 50 to line 100.

The following command will change every occurrence of old to new within the entire file:

:1,$s/old/new/g

You can also use % instead of 1,$ to specify every line in a file. Thus the last command could also be given like this:

:%s/old/new/g

Global replacement is much faster than finding each instance of a string and replacing it individually. Because the command can be used to make many different kinds of changes, and because it is so powerful, we are first going to illustrate simple replacements and then build to complex, context-sensitive replacements.