How to add new highlight mode in GEdit

Here I show common steps to create custom language for GtkSourceView 2.0(GEdit uses it to display code). Then we'll create MBox language and special highlighting for it.

Common steps

1. Make directories for language definition and for new MIME:

$ mkdir -p ~/.local/share/gtksourceview-2.0/language-specs/
$ mkdir -p  ~/.local/share/mime/packages/

2. Create new MIME definition in ~/.local/share/mime/packages/

3. Create .lang file in ~/.local/share/gtksourceview-2.0/language-specs/

4. Update MIME database

$ update-mime-database ~/.local/share/mime

New language for MBox

STEP - 2: New MIME
Actually it should be already in /usr/share/mime/application/mbox.xml. But one would probably override it:

$ cat > ~/.local/share/mime/packages/mbox.xml
<!--
Author:     Ruslan Osmanov
Date:       Tue Apr 19 05:12:58 UTC 2011
Version:    1.0
-->
<mime-info xmlns='http://www.freedesktop.org/standards/shared-mime-info'>
    <mime-type type="application/mbox">
        <comment>MBox Source</comment>
        <magic priority="50">
            <match type="string" offset="0" value="&gt;"/>
        </magic>
        <glob pattern="*.mbox"/>
    </mime-type>
</mime-info>
^D

^D stands for Ctrl + D shortcut.

STEP - 3: New Language Definition

The GNOME Language Definition Reference 2.0[1] and Language Definition v2.0 Tutorial [2] are good start points.
Syntax highlighting styles could be found in /usr/share/gtksourceview-2.0/styles/.

This info is quite enought to implement a new language:

$ cat > ~/.local/share/gtksourceview-2.0/language-specs/mbox.lang
<?xml version="1.0" encoding="UTF-8"?>
<!--
Author: Ruslan Osmanov 
Date:   Mon Apr 18 17:08:52 UTC 2011
Version:   1.0
MBox language spec
-->
<language id="mbox" _name="MBox" version="2.0" _section="Email">
  <metadata>
 <property name="mimetypes">text/x-mbox;application/mbox</property>
 <property name="globs">*.mbox</property>
 <property name="line-comment-start">&gt;</property>
  </metadata>
  <styles>
 <style id="keyword" _name="Keyword" map-to="def:keyword"/>
 <style id="underlined" _name="Underlined" map-to="def:underlined"/>
 <style id="comment" _name="Comment" map-to="def:comment"/>
 <style id="string" _name="A string" map-to="def:string"/>
  </styles>
  <definitions>
 <context id="mbox">
   <include>
  <context id="keyword" style-ref="keyword">
    <prefix>^</prefix>
    <suffix>( |\:)</suffix>
    <keyword>(From|To|Subject)</keyword>
    <!-- Russian -->
    <keyword>(От|Кому|Тема)</keyword>
  </context>
  <context id="link" style-ref="underlined">
   <keyword>\b(ftp|http|https)\:\/\/.*\b</keyword> 
   <keyword>mailto\:.+\@[a-z]+</keyword> 
  </context>
  <context id="comment" style-ref="comment">
    <start>^&gt;</start>
    <end>$</end>
    <include>
   <context ref="link"/>
    </include>
  </context>
  <context id="comment-multiline" style-ref="comment">
    <start>^--</start>
    <include>
   <context ref="def:in-comment"/>
    </include>
  </context>
  <context id="string" end-at-line-end="true" style-ref="string">
    <start>"</start>
    <end>"</end>
  </context>

   </include>
 </context>
  </definitions>
</language>
<!--vim: set ft=xml:-->

^D

Note, ^D stands for Ctrl + D shortcut.

STEP - 4. Update MIME database

$ update-mime-database ~/.local/share/mime

Voila! You now have new MBox language in View - Highlight Mode - Email section.

References

• Language Definition v2.0 Reference
• Language Definition v2.0 Tutorial