# Working with Syntax Highlighting

## Overview

Syntax Highlighting is what makes the editor automatically display text in different styles/colors, depending on the function of the string in relation to the purpose of the file. In program source code for example, control statements may be rendered bold, while data types and comments get different colors from the rest of the text. This greatly enhances the readability of the text, and thus helps the author to be more efficient and productive.

A perl function, rendered with syntax highlighting.

A perl function, rendered with syntax highlighting.

The same perl function, without highlighting.

The same perl function, without highlighting.

Of the two examples, which is easiest to read?

&kate; comes with a flexible, configurable and capable system for doing syntax highlighting, and the standard distribution provides definitions for a wide range of programming languages, markup and scripting languages and other text file formats. In addition you can provide your own definitions in simple &XML; files.

&kate; will automatically detect the right syntax rules when you open a file, based on the &MIME; Type of the file, determined by its extension, or, if it has none, the contents. Should you experience a bad choice, you can manually set the syntax to use from the Documents Highlight Mode menu.

The styles and colors used by each syntax highlight definition, as well as which &MIME;types it should be used for, can be configured using the Highlight page of the Config Dialog.

Syntax highlighting is there to enhance the readability of correct text, but you cannot trust it to validate your text. Marking text for syntax is difficult depending on the format you are using, and in some cases the authors of the syntax rules will be proud if 98% of text gets correctly rendered, though most often you need a rare style to see the incorrect 2%.

You can download updated or additional syntax highlight definitions from the &kate; website by clicking the Download button in the Highlight Page of the Config Dialog.

## The &kate; Syntax Highlight System

This section will discuss the &kate; syntax highlighting mechanism in more detail. It is for you if you want to know know about it, or if you want to change or create syntax definitions.

### How it Works

Whenever you open a file, one of the first things the &kate; editor does is detect which syntax definition to use for the file. While reading the text of the file, and while you type away in it, the syntax highlighting system will analyze the text using the rules defined by the syntax definition and mark in it where different contexts and styles begin and end.

When you type in the document, the new text is analyzed and marked on the fly, so that if you delete a character that is marked as the beginning or end of a context, the style of surrounding text changes accordingly.

The syntax definitions used by the &kate; syntax highlighting system are &XML; files, containing

- Rules for detecting the role of text, organized into context blocks

- Keyword lists

- Style Item definitions

When analyzing the text, the detection rules are evaluated in the order in which they are defined, and if the beginning of the current string matches a rule, the related context is used. The start point in the text is moved to the final point at which that rule matched and a new loop of the rules begins, starting in the context set by the matched rule.

### Rules

The detection rules are the heart of the highlighting detection system. A rule is a string, character or regular expression against which to match the text being analyzed. It contains information about which style to use for the matching part of the text. It may switch the working context of the system either to an explicitly mentioned context or to the previous context used by the text.

Rules are organized in context groups. A context group is used for main text concepts within the format, for example quoted text strings or comment blocks in program source code. This ensures that the highlighting system does not need to loop through all rules when it is not necessary, and that some character sequences in the text can be treated differently depending on the current context.

### Context Styles and Keywords

In some programming languages, integer numbers are treated differently than floating point ones by the compiler (the program that converts the source code to a binary executable), and there may be characters having a special meaning within a quoted string. In such cases, it makes sense to render them differently from the surroundings so that they are easy to identify while reading the text. So even if they do not represent special contexts, they may be seen as such by the syntax highlighting system, so that they can be marked for different rendering.

A syntax definition may contain as many styles as required to cover the concepts of the format it is used for.

In many formats, there are lists of words that represent a specific concept. For example in programming languages, the control statements is one concept, data type names another, and built in functions of the language a third. The &kate; Syntax Highlighting System can use such lists to detect and mark words in the text to emphasize concepts of the text formats.

### Default Styles

If you open a C++ source file, a &Java; source file and an HTML document in &kate;, you will see that even though the formats are different, and thus different words are chosen for special treatment, the colors used are the same. This is because &kate; has a predefined list of Default Styles, that are employed by the individual syntax definitions.

This makes it easy to recognize similar concepts in different text formats. For example comments are present in almost any programming, scripting or markup language, and when they are rendered using the same style in all languages, you do not have to stop and think to identify them within the text.

All styles in a syntax definition use one of the default styles. A few syntax definitions use more styles that there are defaults, so if you use a format often, it may be worth launching the configuration dialog to see if some concepts are using the same style. For example there is only one default style for strings, but as the perl programming language operates with two types of strings, you can enhance the highlighting by configuring those to be slightly different.

## The Highlight Definition &XML; Format

### Overview

This section is an overview of the Highlight Definition &XML; format. It will describe the main components and their meaning and usage, and go into detail with the detection rules.

The formal definition, aka the DTD is stored in the file `language.dtd` which should be installed on your system in the folder `$`KDEDIR`/share/apps/kate/syntax`.

#### Main components of &kate; Highlight Definitions

The General Section

  The General Section contains information on the comment format of the described language, and defines whether keywords are case sensitive.

Highlighting

  The Highlighting section contains all data required to analyze and render the text. This includes:

  ItemDatas

    Contains ItemData elements, each defining a style.

  Keyword lists

    Each list has a name, and may contain any number of items.

  Contexts

    Contains contexts, which again contain the syntax detection rules.

## Highlight Detection Rules

This section describes the syntax detection rules.

Each rule can match zero or more characters at the beginning of the string they are asked to test. If the rule matches, the matching characters are assigned the style or attribute defined by the rule, and a rule may ask that the current context is switched.

The attribute and context attributes are common to all rules.

A rule looks like this:

    <RuleName attribute="(identifier)" context="(identifier|order)" [rule specific attributes] />

The attribute identifies the style to use for matched characters by name or index, and the context identifies the context to use from here.

The attribute can be identified either by name, or by its zero-based index in the ItemDatas group.

The context can be identified by:

- An identifier, currently only its zero-based index in the contexts group.

- An order telling the engine to stay in the current context (`#stay`), or to pop back to a previous context used in the string (`#pop`).

  To go back more steps, the #pop keyword can be repeated: `#pop#pop#pop`

Some rules can have child rules which are then evaluated if and only if the parent rule matched. The entire matched string will be given the attribute defined by the parent rule. A rule with child rules looks like this:

    <RuleName (attributes)>
      <ChildRuleName (attributes) />
      ...
    </RuleName>

Rule specific attributes varies and are described in the following list.

### The Rules in Detail

DetectChar

  Detect a single specific character. Commonly used for example to find the ends of quoted strings.

      <DetectChar char="(character)" (common attributes) />

  The `char` attribute defines the character to match.

Detect2Chars

  Detect two specific characters in a defined order.

      <Detect2Chars char="(character)" char1="(character)" (common attributes) />

  The `char` attribute defines the first character to match, `char1` the second.

AnyChar

  Detect one character of a set of specified characters.

      <AnyChar String="(string)" (common attributes) />

  The `String` attribute defines the set of characters.

StringDetect

  Detect an exact string.

      <StringDetect String="(string)" [insensitive="TRUE|FALSE;"] (common attributes) />

  The `String` attribute defines the string to match. The `insensitive` attribute defaults to `FALSE` and is fed to the string comparison function. If the value is `TRUE` insensitive comparing is used.

RegExpr

  Matches against a regular expression.

      <RegExpr String="(string)" [insensitive="TRUE|FALSE;"] [minimal="TRUE|FALSE"] (common attributes) />

  The `String` attribute defines the regular expression.

  `insensitive` defaults to `FALSE` and is fed to the regular expression engine.

  `minimal` defaults to `FALSE` and is fed to the regular expression engine.

  Because the rules are always matched against the beginning of the current string, a regular expression starting with a caret (`^`) indicates that the rule should only be matched against the start of a line.

  See Regular Expressions for more information on those.

Keyword

  Detect a keyword from a specified list.

      <keyword String="(list name)" (common attributes) />

  The `String` attribute identifies the keyword list by name. A list with that name must exist.

Int

  Detect an integer number.

      <Int (common attributes) />

  This rule has no specific attributes. Child rules are typically used to detect combinations of `L` and `U` after the number, indicating the integer type in program code.

Float

  Detect a floating point number.

      <Float (common attributes)
      />

  This rule has no specific attributes.

HlCOct

  Detect an octal point number representation.

      <HlCOct (common attributes) />

  This rule has no specific attributes.

HlCHex

  Detect a hexadecimal number representation.

      <Int (common attributes) />

  This rule has no specific attributes.

HlCStringChar

  Detect an escaped character.

      <HlCStringChar (common attributes)
      />

  This rule has no specific attributes.

  It matches letteral representations of invisible characters commonly used in program code, for example `\n` (newline) or `\t` (TAB).

  The following characters will match if they follow a backslash (`\`): `abefnrtv"'?`. Additionally, escaped hexadecimal numbers like for example `\xff` and escaped octal numbers, for example `\033` will match.

RangeDetect

  Detect a string with defined start and end characters.

      <RangeDetect char="(character)"  char1="(character)" (common attributes) />

  `char` defines the character starting the range, `char2` the character ending the range.

  Usefull to detect for example small quoted strings and the like, but note that since the hl engine works on one line at a time, this will not find strings spanning over a line break.

LineContinue

  Matches at end of line.

      <LineContinue (common attributes) />

  This rule has no specific attributes.

  This rule is useful for switching context at end of line.