Syntax highlighting specification file for lex/flex (the lexical analyzer often used alongside bison/yacc), to be used in gtksourceview (used by Gedit). Should do its work :)

lex.lang

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

<!--

 This file is part of GtkSourceView

 Author: Marcello Pogliani
 Copyright (C) 2012 Marcello Pogliani

 GtkSourceView is free software; you can redistribute it and/or
 modify it under the terms of the GNU Lesser General Public
 License as published by the Free Software Foundation; either
 version 2.1 of the License, or (at your option) any later version.

 GtkSourceView is distributed in the hope that it will be useful,
 but WITHOUT ANY WARRANTY; without even the implied warranty of
 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 Lesser General Public License for more details.

 You should have received a copy of the GNU Lesser General Public
 License along with this library; if not, write to the Free Software
 Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA

-->

<language id="lex" _name="Lex" _section="Others" version="2.0">
  <metadata>
    <property name="globs">*.l;*.lex;*.flex</property>
    <property name="line-comment-start">//</property>
    <property name="block-comment-start">/*</property>
    <property name="block-comment-end">*/</property>
  </metadata>

  <styles>
    <style id="token" _name="Token" map-to="def:keyword" />
    <style id="keyword" _name="Keyword" map-to="def:keyword" />
    <style id="regexp" _name="Pattern" map-to="def:identifier" />
    <style id="expression" _name="Expression" map-to="def:type" />
  </styles>

  <definitions>

    <define-regex id="identifier">[a-zA-Z_.][a-zA-Z0-9_.]*</define-regex>
    <define-regex id="start-cond">&lt;\%{identifier}(,\%{identifier})*&gt;</define-regex>

    <!-- blocks for embedded C code -->
    <context id="inline-c">
      <start>^%{</start>
      <end>^%}</end>
      <include>
        <context sub-pattern="0" where="start" style-ref="keyword"/>
        <context sub-pattern="0" where="end" style-ref="keyword"/>
        <context ref="c:c"/>
      </include>
    </context>

    <context id="top-block">
      <start>^%top{</start>
      <end>^}</end>
      <include>
        <context sub-pattern="0" where="start" style-ref="keyword"/>
        <context sub-pattern="0" where="end" style-ref="keyword"/>
        <context ref="c:c"/>
      </include>
    </context>

    <context id="c-with-brackets">
      <include>
        <context>
          <start>{</start>
          <end>}</end>
          <include>
            <context ref="c-with-brackets"/>
          </include>
        </context>
        <context ref="c:c"/>
      </include>
    </context>

    <context id="indented-lines-c-code">
        <start>^(?=[ \t])</start>
        <end>$</end>
        <include><context ref="c:c" /></include>
    </context>

    <!-- (1) definition section (before the first %%) -->
    <context id="definitions">
        <include>
            <context ref="inline-c"/> <!-- %{ ... %} -->
            <context ref="top-block"/> <!-- %top -->
            <context ref="indented-lines-c-code" /> <!-- indented lines are copied verbatim to output, hence they're pure C code -->
            <context ref="option-or-scope"/>
            <context ref="def:c-like-comment"/>
            <context ref="def:c-like-comment-multiline"/>
            <context ref="def:c-like-close-comment-outside-comment"/>
            <context ref="definition"/>
        </include>
    </context>

    <context id="definition" style-ref="expression">
        <start>^\%{identifier}</start>
        <end>$</end>
        <include><context sub-pattern="0" where="start" style-ref="token"/></include>
    </context>

    <context id="option-or-scope" style-ref="keyword">
        <prefix>^%</prefix>
        <keyword>option</keyword>
        <keyword>s</keyword>
        <keyword>x</keyword>
        <keyword>pointer</keyword>
        <keyword>array</keyword>
    </context>

    <!-- (2) rule section (after the first %%) -->
    <context id="rule-section">
        <start>^%%</start>
	    <include>
                <context sub-pattern="0" where="start" style-ref="token"/>
                <context ref="rule"/>
                <context ref="indented-lines-c-code"/>
                <context ref="inline-c"/>
                <context ref="user-code"/>
	    </include>
    </context>   

    <context id="rule">
        <start>^(?=[^% \t])</start>
        <end>$</end>
        <include>
        <context id="pattern" style-ref="expression">
            <start>^</start>
            <end>[ \t]</end>
            <include>
               <context ref="round-brackets" />
               <context ref="square-brackets" />
               <context>
                   <start>"</start>
                   <end>"</end>
                   <include><context ref="escaped-char" /></include>
               </context>
               <context ref="start-condition" /> <!-- <SOMETHING>pattern -->
               <context ref="start-condition-block" /> <!-- <SOMETHING>{ block } -->
               <context ref="escaped-char" />
            </include>
        </context>
        <context ref="c-with-brackets"/>
        </include>
    </context>

    <context id="start-condition" style-ref="keyword"> 
        <match>^\%{start-cond}(?=[^{])</match>
    </context>

    <context id="start-condition-block"> 
        <start>(^\%{start-cond}){</start>
        <end>}</end>
        <include>
            <context sub-pattern="1" where="start" style-ref="keyword"/>
            <context ref="c-with-brackets" />
        </include>
    </context>

    <context id="escaped-char">
        <match>\\.</match>
    </context>

    <context id="square-brackets">
        <start>\[</start>
        <end>\]</end>
        <include>
            <context ref="escaped-char" />
            <context ref="square-brackets" />
        </include>
    </context>

    <context id="round-brackets">
        <start>\(</start>
        <end>\)</end>
        <include>
            <context ref="escaped-char" />
            <context ref="round-brackets" />
        </include>
    </context>

    <!-- (3) user code section, this is pure C -->
    <context id="user-code">
        <start>^%%</start>
        <include>
            <context sub-pattern="0" where="start" style-ref="token"/>
            <context ref="c:c"/>
        </include>
    </context>

    <!-- Main context -->
    <context id="lex" class="no-spell-check">
      <include>
	<context ref="definitions"/>
	<context ref="rule-section"/>
      </include>
    </context>

  </definitions>
</language>