chapters/31.markdown @ 15621a1cf5df

Basic Regular Expressions
=========================

Vim is a text editor, which means that a great deal of your Vimscript code will
be dedicated to working with text.  Vim has powerful support for regular
expressions, but as usual there are some quirks.

Type the following text into a buffer:

    :::text
    max = 10

    print "Starting"

    for i in range(max):
        print "Counter:", i

    print "Done"

This is the text we'll use to experiment with Vimscript's regex support.  It
happens to be Python code, but don't worry if you don't know Python.  It's just
an example.

I'm going to assume that you know the basics of regular expressions.  If you
don't you should stop reading this book and start reading [Learn Regex the Hard
Way][regex] by Zed Shaw.  Come back when you're done with that.

[regex]: http://regex.learncodethehardway.org/

Highlighting
------------

Before we start we need to turn on search highlighting so we can see what we're
doing.  Run the following command:

    :::vim
    :set hlsearch incsearch

`hlsearch` tells Vim to highlight all matches in a file when you perform
a search, and `incsearch` tells Vim to highlight the *next* match while you're
still typing out your search pattern.

Searching
---------

Put your cursor at the top of the file and run the following command:

    :::vim
    /print

As you type in each letter, Vim will start highlighting them in the first line.
When you press return to execute the search *all* the instances of `print` will
be highlighted and your cursor will be moved to the next match.

Now try running the following command:

    :::vim
    :execute "normal! gg/print\<cr>"

This will go to the top of the file and perform a search for `print`, putting us
at the first match.  It does this using `:execute "normal! ..."` which we saw in
the previous chapter.

To get to the second match in the file you can just add more commands onto the
end of the string.  Run this command:

    :::vim
    :execute "normal! gg/print\<cr>n"

Vim will put the cursor on the second `print` in the buffer (and all the matches
will be highlighted).

Let's try going in the opposite direction.  Run this command:

    :::vim
    :execute "normal! G?print\<cr>"

This time we move to the bottom of the file with `G` and use `?` to search
backward instead of forward.

All of these searching commands should be familiar -- we're mostly going over
them to get you used to the `:execute "normal! ..."` idiom, because it will let
you do anything you know how to do in vanilla Vim in your Vimscript code.

Magic
-----

The `/` and `?` commands actually take regular expressions, not just literal
characters.  Run the following command:

    :::vim
    :execute "normal! gg/for .+ in .+:\<cr>"

Vim complains that the pattern is not found!  I told you that Vim supports
regular expressions in searches, so what's going on?  Try the following command:

    :::vim
    :execute "normal! gg/for .\\+ in .\\+:\<cr>"

This time Vim highlights the "for" loop as we expected in the first place.  Take
a minute and try to think about what exactly changed before moving on.  Remember
that `execute` takes a String.

The answer is that there are two reasons we needed to write the command like we
did:

* First, `execute` takes a String, so the double backslashes we used turn into
  single backslashes by the time they get to `normal!`.
* Vim has *four* different "modes" of parsing regular expressions!  The default
  mode requires a backslash before the `+` character to make it mean "1 or more
  of the preceding character" instead of "a literal plus sign".

You can see this a bit easier by just running the search in Vim directly.  Type
the following command and press return:

    :::vim
    /print .\+

You can see the `\+` working its magic now.  The double backslashes were only
used because we were passing the pattern as a String to `execute`.

Literal Strings
---------------

As we mentioned in the chapter on Strings, Vim allows you to use single quotes
to define a "literal string" that passes through characters directly.  For
example, the string `'a\nb'` is four characters long.

Can we use literal strings to avoid having to type those double backslashes?
Think about this for a minute or two before you move on, because the answer is
a bit more complicated that you might think.

Try running the following command (note the single quotes and single backslashes
this time):

    :::vim
    :execute 'normal! gg/for .\+ in .\+:\<cr>'

Vim moves you to the top of the file but doesn't move you to the first match.
Is this what you expected?

The command doesn't work because we need the `\<cr>` in the pattern to be
escaped into a real carriage return character, which tells the search command to
actually run.  Because we're in a literal string, it's the equivalent of typing
`/for .\+ in .\+:\<cr>` in vanilla Vim, which obviously isn't what we want.

All hope is not lost, though!  Remember that Vim allows you to concatenate
strings, so for larger commands we can use this to split apart the string into
easier to read chunks.  Run the following command:

    :::vim
    :execute "normal! gg" . '/for .\+ in .\+:' . "\<cr>"

This concatenates the three smaller strings before sending them to `execute`,
and lets us use a literal string for the regex while using normal strings for
everything else.

Very Magic
----------

You may be wondering about Vimscript's four different modes of regex parsing and
how they're different from the regular expressions you're used to from languages
like Python, Perl or Ruby.  You can read their documentation if you really want
to, but if you want the sane, easy solution just read on.

Run the following command:

    :::vim
    :execute "normal! gg" . '/\vfor .+ in .+:' . "\<cr>"

We've split the pattern out from the rest of the command into its own literal
string again, and this time we started the pattern with `\v`.  This tells Vim to
use its "very magic" regex parsing mode, which is pretty much the same as you're
used to in any other programming language.

If you simply start all of your regular expressions with `\v` you'll never need
to worry about Vimscript's three other crazy regex modes.

Exercises
---------

Read `:help magic` carefully.

Read `:help pattern-overview` to see the kinds of things Vim regexes support.
Stop reading after the character classes.

Read `:help :match`.  Try running the `:match Error /\v.../` command a few times
by hand.

Edit your `~/.vimrc` file to add a mapping that will use `match` to highlight
trailing whitespace as an error.  A good key to use might be `<leader>w`.

Add another mapping that will clear the match (perhaps `<leader>W`).

Add a normal mode mapping that will automatically insert the `\v` for you
whenever you begin a search.  If you're stuck remember that Vim's mappings are
extremely simple and you just need to tell it which keys to press when you use
the mapped key.

Add the `hlsearch` and `incsearch` options to your `~/.vimrc` file, set however
you prefer.

Read `:help nohlsearch`.  Note that this is a *command* and *not* the "off mode"
setting of `hlsearch`!

Add a mapping to "stop highlighting items from the last search" to your
`~/.vimrc` file.