Custom highlighting for gtksourceview not working

I am trying to write my own syntax highlighting for gtksourceview. If it's done it should highlight guitar chords in a text file. This is my unfinished file:

<?xml version="1.0" encoding="UTF-8"?>
<language id="tab" _name="Tab" version="2.0" _section="Others">
  <metadata>
    <property name="mimetypes">text/plain</property>
    <property name="globs">*.txt;*.tab</property>
  </metadata>

  <styles>
    <style id="chord" _name="Chord" foreground="blue" bold="true"/>
  </styles>

  <definitions>

    <context id="chord" style-ref="chord" class="no-spell-check">
      <match>[A-H]</match>
    </context>

    <context id="tab">
      <include>
        <context ref="chord"/>
      </include>
    </context>

  </definitions>
</language>

I would expect every capital A to H to be blue and bold in the highlighted text. Is saved the file to /usr/share/gtksourceview-3.0/language-specs and I am able to select it in GEdit. But nothing is highlighted in the text.

Can anyone tell me what I am doing wrong?