Find and Replace with Emacs

Xah Lee, 2008-03

This page shows you how to use emacs to do find and replace operations, and tells you how to do case-sensitive or case-insensitive match or replacement, and how to force captured regex text pattern into upper or lower case.

Emacs's Find and Replace Commands

Here are the emacs find and replace commands. These are also under the graphical menu “Edit‣Replace”.

For example, to use query-replace, type “Alt+%”, then type your search string, then type your replacement string.

When the query commands stops to ask you for confirmation, type “y” (or space bar) to do the replacement, type “n” to skip, type “!” to do all remaining replacements without asking, type “q” to exit.

For tutorial on how to use dired-do-query-replace-regexp, see Interactively Find and Replace String Patterns on Multiple Files.

Emacs also have commands replace-string and replace-regexp. They are the non-interactive versions of query-replace and query-replace-regexp. They do all replacements in one-shot without asking confirmation for each replacement.

Case Sensitivity in Search String

By default, search is not case sensitive; however, if your search string contains a capital letter, search is automatically case sensitive.

To make your search absolutely case-sensitive, use the menu “Options‣Case-Insensitive Search”. The menu is a toggle. A checkmark in front of the menu means it is on.

(Technically, the case sensitivity is controlled by the variable case-fold-search. You can set this variable to true (t) or false (nil) by typing “M-x set-variable”, then the vaiable name, then give a value “t” or “nil”. To see what the current value is, type “M-x describe-variable” then give the variable name. There is also a command toggle-case-fold-search, which toggles the value of case-fold-search)

Case in Replacement String

By default, the case of the replaced text is smartly dependent on the matched text. For example, suppose your search string is “here”, and your replacement string is “dragon” (and assume you are using default emacs setup so that it will match both “here”, “Here”, “HERE”). Now, when emacs found “here”, the replacement will be “dragon”, when emacs found “Here”, the replacement will be “Dragon”, when emacs found “HERE”, the replacement will be “DRAGON”.

If you want the letter case of your replacement string be literal, you need to set the variable case-replace to “t”. Type “Alt+x set-variable” then the variable name, then give it a value of “t”.

Force Case of Matched Text

Sometimes you need to do regex search to find a pattern, and have the case of the pattern be changed. For example, suppose in your html code you have:

<p>once upon a time ...</p>

<P>There is a dragon who lived in ...</P>

<p>princess Tana is still waiting ...</p>

You want to make sure that all paragraphs starts with a capital letter. So, you use a pattern that catches the first letter after <p>, like this “<p>\([a-z]\)”. By default, emacs will match both “<P>” and “<p>”.

To make your captured pattern upper case, give your replacement string this expression: “<p>\,(upcase \1)”. The “\,” tells emacs that what follows should be a lisp expression. The “(upcase \1)” is a lisp expression. The “upcase” is a lisp function and the “\1” means the 1st captured string in your regex pattern. If you want lower case, use “downcase” in place of “upcase”.

For a more complex example in using the “\,” in replacement, see: Lisp Lesson: Regex Replace with a Function.

Reference: (info "(emacs)Search").

Related essays:

• Interactively Find and Replace String Patterns on Multiple Files
• Elisp Language Basics
• Emacs Regex
• Perl-Python: Find and Replace By Regex Text Patterns

Page created: 2007-08.
© 2007 by Xah Lee.