text editor is a type of computer program that edits plain text. Such programs are sometimes known as " notepad" software, following the naming of Microsoft Notepad.
Plain text exclusively consists of character representation. Each character is represented by one, two or four bytes, in accordance to character encoding conventions, such as ASCII or Unicode. These conventions define many printable characters but also non-printing characters that control the flow of the text, such space, newline and page break but plain text contains no other information about the text itself, not even the character encoding convention employed. Plain text is stored in , although text files do not exclusively store plain text. In the early days of computers, plain text was displayed using a monospace font, so horizontal alignment and columnar formatting were sometimes done using whitespace characters. For compatibility reason, this tradition did not change.
Rich text, on the other hand, may contain metadata, character formatting data (e.g. typeface, size, weight and style), paragraph formatting data (e.g. indentation, alignment, letter and word distribution, and space between lines or other paragraphs) and page specification data (e.g. size, margin and reading direction). Rich text can become very complex. Rich text can be saved in binary format (e.g. DOC), text files adhering to a markup language (e.g. RTF or HTML) or a hybrid form of both (e.g. Office Open XML).
Text editors are intended to open and save text files containing either plain text or anything that can be interpretted as plain text, including the markup for rich text or the markup for something else (e.g. SVG).
The first text editors were Line editor oriented to teleprinter- or typewriter-style terminals without displays. Commands (often a single keystroke) effected edits to a file at an imaginary insertion point called the "cursor". Edits were verified by typing a command to print a small section of the file, and periodically by printing the entire file. In some line editors, the cursor could be moved by commands that specified the line number in the file, text strings (context) for which to search, and eventually regular expressions. Line editors were major improvements over keypunching. Some line editors could be used by keypunch; editing commands could be taken from a deck of cards and applied to a specified file. Some common line editors supported a "verify" mode in which change commands displayed the altered lines.
When computer terminals with video screens became available, Visual editor (sometimes called just "screen editors") became common. One of the earliest full-screen editors was O26, which was written for the operator console of the CDC 6000 series computers in 1967. Another early full-screen editor was vi. Written in the 1970s, it is still a standard editor on Unix and Linux operating systems. Also written in the 1970s was the UCSD Pascal Screen Oriented Editor, which was optimized both for indented source code as well as general text. Emacs, one of the first open source and free software projects, is another early full-screen or real-time editor, one that was ported to many systems. A full-screen editor's ease-of-use and speed (compared to the line-based editors) motivated many early purchases of video terminals.
The core data structure in a text editor is the one that manages the string (sequence of characters) or list of Storage record that represents the current state of the file being edited. While the former could be stored in a single long consecutive array of characters, the desire for text editors that could more quickly insert text, delete text, and undo/redo previous edits led to the development of more complicated sequence data structures. Charles Crowley. "Data Structures for Text Sequences". Section "Introduction". A typical text editor uses a gap buffer, a linked list of lines (as in PaperClip), a piece table, or a rope, as its sequence data structure.
Most word processors can read and write files in plain text format, allowing them to open files saved from text editors. Saving these files from a word processor, however, requires ensuring the file is written in plain text format, and that any text encoding or BOM settings won't obscure the file for its intended use. Non-WYSIWYG word processors, such as WordStar, are more easily pressed into service as text editors, and in fact were commonly used as such during the 1980s. The default file format of these word processors often resembles a markup language, with the basic format being plain text and visual formatting achieved using non-printing control characters or . Later word processors like Microsoft Word store their files in a binary format and are almost never used to edit plain text files.
Some text editors can edit unusually large files such as logfile or an entire database placed in a single file. Simpler text editors may just read files into the computer's main memory. With larger files, this may be a slow process, and the entire file may not fit. Some text editors do not let the user start editing until this read-in is complete. Editing performance also often suffers in nonspecialized editors, with the editor taking seconds or even minutes to respond to keystrokes or navigation commands. By only storing the visible portion of large files in memory, editing performance improves.
Some editors are programmable, meaning they can be customized for specific uses. One motive for customizing is to make a text editor use the commands of another text editor with which the user is more familiar, or to duplicate missing functionality the user has come to depend on. Software developers often use editor customizations tailored to the programming language or development environment they are working in. The programmability of some text editors is limited to enhancing the core editing functionality of the program, but Emacs can be extended far beyond editing text files—for web browsing, reading email, online chat, managing files or playing games. Emacs can even emulate Vi, its rival in the traditional editor wars of Unix philosophy.
An important group of programmable editors uses REXX as a scripting language. These "orthodox editors" contain a "command line" into which commands and macros can be typed and text lines into which line commands and macros can be typed. Most such editors are derivatives of ISPF EDIT or of XEDIT, IBM's flagship editor for VM/SP through z/VM. Among them are THE, KEDIT, X2, Uni-edit, and SEDIT.
A text editor written or customized for a specific use can determine what the user is editing and assist the user, often by autocomplete programming terms and showing with relevant documentation. Many text editors for software developers include source code syntax highlighting and automatic indent style to make programs easier to read and write. Programming editors often let the user select the name of an include file, function or variable, then jump to its definition. Some also allow for easy navigation back to the original section of code by storing the initial cursor location or by displaying the requested definition in a popup window or temporary buffer. Some editors implement this ability themselves, but often an auxiliary utility like ctags is used to locate the definitions.
Programmable editors can usually be enhanced to perform any or all of these functions, but simpler editors focus on just one, or, like gPHPedit, are targeted at a single programming language.