6.5.2. Switching Items in a Database

Name: Feld, Ray; Areas: PC, UNIX; Phone: 123-4567
Name: Joy, Susan S.; Areas: Graphics; Phone: 999-3333

The name of each field ends with a colon, and each field is separated by a semicolon. Using the top line as an example, you want to change Feld, Ray to Ray Feld. We'll present some commands that look promising but don't work. After each command, we show you the line the way it looked before the change and after the change.

:%s/: \(.*\), \(.*\);/: \2 \1;/

Name: Feld, Ray; Areas: PC, UNIX; Phone: 123-4567	Before
Name: UNIX Feld, Ray; Areas: PC; Phone: 123-4567	After

We've highlighted the contents of the first hold buffer in bold and the contents of the second hold buffer in italic. Note that the first hold buffer contains more than you want. Since it was not sufficiently restricted by the pattern that follows it, the hold buffer was able to save up to the second comma. Now you try to restrict the contents of the first hold buffer:

:%s/: \(....\), \(.*\);/: \2 \1;/

Name: Feld, Ray; Areas: PC, UNIX; Phone: 123-4567	Before
Name: Ray; Areas: PC, UNIX Feld; Phone: 123-4567	After

Here you've managed to save the last name in the first hold buffer, but now the second hold buffer will save anything up to the last semicolon on the line. Now you restrict the second hold buffer, too:

:%s/: \(....\), \(...\);/: \2 \1;/

Name: Feld, Ray; Areas: PC, UNIX; Phone: 123-4567	Before
Name: Ray Feld; Areas: PC, UNIX; Phone: 123-4567	After

This gives you what you want, but only in the specific case of a four-letter last name and a three-letter first name. (The previous attempt included the same mistake.) Why not just return to the first attempt, but this time be more selective about the end of the search pattern?

:%s/: \(.*\), \(.*\); Area/: \2 \1; Area/

Name: Feld, Ray; Areas: PC, UNIX; Phone: 123-4567	Before
Name: Ray Feld; Areas: PC, UNIX; Phone: 123-4567	After

This works, but we'll continue the discussion by introducing an additional concern. Suppose that the Area field isn't always present or isn't always the second field. The above command won't work on such lines.

We introduce this problem to make a point. Whenever you rethink a pattern match, it's usually better to work toward refining the variables (the metacharacters), rather than using specific text to restrict patterns. The more variables you use in your patterns, the more powerful your commands will be.

In the current example, think again about the patterns you want to switch. Each word starts with an uppercase letter and is followed by any number of lowercase letters, so you can match the names like this:

[A-Z][a-z]*

A last name might also have more than one uppercase letter (McFly, for example), so you'd want to search for this possibility in the second and succeeding letters:

[A-Z][A-Za-z]*

It doesn't hurt to use this for the first name, too (you never know when McGeorge Bundy will turn up). Your command now becomes:

:%s/: \([A-Z][A-Za-z]*\), \([A-Z][A-Za-z]*\);/: \2 \1;/

Quite forbidding, isn't it? It still doesn't cover the case of a name like Joy, Susan S. Since the first-name field might include a middle initial, you need to add a space and a period within the second pair of brackets. But enough is enough. Sometimes, specifying exactly what you want is more difficult than specifying what you don't want. In your sample database, the last names end with a comma, so a last-name field can be thought of as a string of characters that are not commas:

[^,]*

This pattern matches characters up until the first comma. Similarly, the first-name field is a string of characters that are not semicolons:

[^;]*

Putting these more efficient patterns back into your previous command, you get:

:%s/: \([^,]*\), \([^;]*\);/: \2 \1;/

The same command could also be entered as a context-sensitive replacement. If all lines begin with Name, you can say:

:g/^Name/s/: \([^,]*\), \([^;]*\);/: \2 \1;/

You can also add an asterisk after the first space, in order to match a colon that has extra spaces (or no spaces) after it:

:g/^Name/s/: *\([^,]*\), \([^;]*\);/: \2 \1;/

6.5.3. Using :g to Repeat a Command

:g/mg[ira]box/s/box/square/g
:g/^$/d

Instead, he demonstrates a technique by which you can repeat ex commands some arbitrary number of times. For example, suppose you want to place ten copies of lines 12 through 17 of your file at the end of your current file. You could type:

:1,10g/^/ 12,17t$

This is a very unexpected use of g, but it works! The g command selects line 1, executes the specified t command, then goes on to line 2, to execute the next copy command. When line 10 is reached, ex will have made ten copies.

6.5.4. Collecting Lines

301  Part 2
302  Capability Reference
303  .LP
304  Chapter 7
305  Introduction to the Capabilities
306  This and the next three chapters ...

400  ... and a complete index at the end.
401  .LP
402  Chapter 8
403  Screen Dimensions
404  Before you can do anything useful
405  on the screen, you need to know ...

555  .LP
556  Chapter 9
557  Editing the Screen
558  This chapter discusses ...

821  .LP
822  Part 3:
823  Advanced Features
824  .LP
825  Chapter 10

The chapter numbers appear on one line, their titles appear on the line below, and the chapter text (highlighted for emphasis) begins on the line below that. The first thing you'd like to do is copy the beginning line of each chapter, sending it to an already existing file called begin.

Here's the command that does this:

:g /^Chapter/ .+2w >> begin

You must be at the top of your file before issuing this command. First you search for Chapter at the start of a line, but then you want to run the command on the beginning line of each chapter—the second line below Chapter. Because a line beginning with Chapter is now selected as the current line, the line address .+2 will indicate the second line below it. The equivalent line addresses +2 or ++ work as well. You want to write these lines to an existing file named begin, so you issue the w command with the append operator >>.

Suppose you want to send the beginnings of chapters that are only within Part 2. You need to restrict the lines selected by g, so you change your command to this:

:/^Part 2/,/^Part 3/g /^Chapter/ .+2w >> begin

Here, the g command selects the lines that begin with Chapter, but it searches only that portion of the file from a line starting with Part 2 through a line starting with Part 3. If you issue the above command, the last lines of the file begin will read as follows:

This and the next three chapters ...
Before you can do anything useful
This chapter discusses ...

These are the lines that begin Chapters 7, 8, and 9.

In addition to the lines you've just sent, you'd like to copy chapter titles to the end of the document, in preparation for making a table of contents. You can use the vertical bar to tack a second command after your first command, like so:

:/^Part 2/,/^Part 3/g /^Chapter/ .+2w >> begin | +t$

Remember that with any subsequent command, line addresses are relative to the previous command. The first command has marked lines (within Part 2) that start with Chapter, and the chapter titles appear on a line below such lines. Therefore, to access chapter titles in the second command, the line address is + (or the equivalents +1 or .+1). Then use t$ to copy the chapter titles to the end of the file.

As these examples illustrate, thought and experimentation may lead you to some unusual editing solutions. Don't be afraid to try things! Just be sure to back up your file first!