Книга: Learning GNU Emacs, 3rd Edition

3.1 Different Kinds of Searches

While you're editing, you frequently want to find something you've already typed. Rather than hunt through the file trying to find what you're looking for, virtually all editors provide some kind of search feature that lets you look for a particular text string. Emacs is no exception to the rule. It supplies a search command—in fact, it provides a dizzying array of search commands. Here's a quick summary of the different kinds of searches that are available:

Simple search

You give Emacs a search string, and it finds the next occurrence. You will find this search in almost any editor.

Incremental search

With incremental search, Emacs starts to search the file as soon as you type the first character of a search string. It continues to search as you type more characters.

Word search

A word search is like a simple search, except that Emacs searches only for full words and phrases. For example, if you are searching for the word hat, you don't have to worry about finding the word that. A word search is also useful when you need to find a phrase that is spread across two lines.

Regular expression search

To search for patterns, you can use a regular expression search. For example, if you wanted to find all instances of B1 and B2, you could search for them using the regular expression B[12]. However, regular expressions can be extremely complex. We'll give a brief introduction to this topic here; it is discussed more fully in Chapter 11.

Incremental regular expression search

This search procedure is a combination of an incremental search and a regular expression search.

You can search forward or backward. Searches can be either case-sensitive, meaning that Emacs considers upper- and lowercase letters to be different (i.e., the words This and this are different) or case-insensitive, in which upper- and lowercase are not differentiated (i.e., This and this are equivalent). By default, searches are case-insensitive, with upper- and lowercase letters considered to be the same. One exception: if you type any uppercase letters, Emacs makes the whole search string case-sensitive; it assumes you are looking for something precise since you've made the extra effort to type some letters in uppercase.

Replacement operations are closely related to searches. As with searches, Emacs offers you several different flavors:

Simple search and replace

In this procedure, Emacs replaces all occurrences of one string with another. Usually, this is too radical a solution and can have unintended results. Try query-replace instead.

Query-replace

With query-replace, Emacs conditionally replaces a string throughout a file. Emacs finds all occurrences of the search string, and for each one it asks you whether or not to perform the replacement. This type of replacement is useful if you need to change some, but not all, instances of a word or phrase throughout a file.

Regular expression replace

Regular expression replacement uses the powerful pattern matching facility of the same name to find strings and replace them.

So now you know what you'll be looking at. Don't be intimidated by the wealth of searches that are available. In practice, you'll probably settle on one search command and one replace command and use these for 99 percent of your work. For example, we use incremental search and query-replace most of the time. If you're a writer, you may use word search all the time; if you're a programmer, you might want a regular expression search. If you're just beginning, you may want to learn incremental search and read the rest of this chapter later. However, if you know what's available, you'll be able to make use of the other search commands when they become useful.

Оглавление книги