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Syntax highlighting is a feature of text editors that are used for programming, scripting, or markup languages, such as HTML. The feature displays text, especially source code, in different colors and fonts according to the category of terms. This feature facilitates writing in a structured language such as a programming language or a markup language as both structures and syntax errors are visually distinct. Highlighting does not affect the meaning of the text itself; it is intended only for human readers.
Contents
Syntax highlighting is a form of secondary notation, since the highlights are not part of the text meaning, but serve to reinforce it. Some editors also integrate syntax highlighting with other features, such as spell checking or code folding, as aids to editing which are external to the language.
Practical benefits
Syntax highlighting is one strategy to improve the readability and context of the text; especially for code that spans several pages. The reader can easily ignore large sections of comments or code, depending on what they are looking for. Syntax highlighting also helps programmers find errors in their program. For example, most editors highlight string literals in a different color. Consequently, spotting a missing delimiter is much easier because of the contrasting color of the text. Brace matching is another important feature with many popular editors. This makes it simple to see if a brace has been left out or locate the match of the brace the cursor is on by highlighting the pair in a different color.
A study published in the conference PPIG evaluated the effects of syntax highlighting on the comprehension of short programs, finding that the presence of syntax highlighting significantly reduces the time taken for a programmer to internalise the semantics of a program. Additionally, data gathered from an eye-tracker during the study suggested that syntax highlighting enables programmers to pay less attention to standard syntactic components such as keywords.
Support in text editors
Some text editors can also export the color markup in a format that is suitable for printing or for importing into word-processing or other kinds of text-formatting software; for instance an HTML, colorized LaTeX, PostScript or RTF version of its syntax highlighting. There are several syntax highlighting libraries or "engines" that can be used in other applications, but are not complete programs in themselves, e.g., GeSHi for PHP.
For editors that support more than one language, the user can usually specify the language of the text, such as C, LaTeX, HTML, or the text editor can automatically recognize it based on the file extension or by scanning contents of the file. This automatic language detection presents potential problems. For example, a user may want to edit a document containing:
In these cases, it is not clear what language to use, and a document may not be highlighted or be highlighted incorrectly.
Syntax elements
Most editors with syntax highlighting allow different colors and text styles to be given to dozens of different lexical sub-elements of syntax. These include keywords, comments, control-flow statements, variables, and other elements. Programmers often heavily customize their settings in an attempt to show as much useful information as possible without making the code difficult to read.
Examples
Below is a comparison of a