LinuxQuestions.org - Custom syntax highlighting on Gedit

I want the plus signs (+) to be highlighted everywere in the text. How does the .lang-file have to look like? The name for this highlight should be 'ABC' and it includes all '.abc'-files.

I tried to start like this (a lot of it is just copied and has to be customized but I do not know how):

Code:

<?xml version="1.0" encoding="UTF-8"?>

<language id="abc" _name="ABC" version="2.0" _section="Markup">

  <metadata>

<property name="mimetypes">text/plain</property>

<property name="globs">*.abc</property>

  </metadata>

  <styles>

<style id="indent2" _name="2x indented"  map-to="def:headword" />

  </styles>



  <definitions>

    <context id="headword" style-ref="comment" class="comment" class-disabled="no-spell-check">

  <start>+</start>

      <include>

        <context ref="def:headword"/>

      </include>

    </context>

  </definitions>

</language>