YAML syntax grammar for AvalonEdit

YAML-Mode.xshd

<?xml version="1.0"?>
<!-- Imperfect syntax grammar for YAML by Tobias Kiertscher <dev@mastersign.de> -->
<SyntaxDefinition xmlns="http://icsharpcode.net/sharpdevelop/syntaxdefinition/2008"
                  name="YAML" extensions=".yaml,.yml">

  <Color name="Comment" foreground="LightSlateGray" fontStyle="italic" />
  <Color name="String" foreground="DarkGreen" fontWeight="normal" />
  <Color name="Array" foreground="DarkMagenta" fontWeight="bold" />
  <Color name="MultilineStringIndicator" foreground="DarkMagenta" fontWeight="bold" />
  <Color name="MapKey" foreground="Firebrick" />
  <Color name="Keyword" foreground="DarkCyan" />
  <Color name="Number" foreground="RoyalBlue" />
  <Color name="Json" foreground="SlateBlue" />
  <Color name="Punctuation" foreground="MediumSlateBlue" />

  <RuleSet>
    <Span color="Comment" begin="#" />

    <Span color="MultilineStringIndicator" multiline="true">
      <Begin>(?&lt;=\:)\s*[&gt;|][+-]?\d*</Begin>
      <End>^$</End>
      <RuleSet>
        <Rule color="String">.+</Rule>
      </RuleSet>
    </Span>

    <Import ruleSet="Literals" />
    <Span ruleSet="JsonExpression">
      <Begin>(?&lt;=\:)\s(?=\[|\{)</Begin>
    </Span>

    <Rule color="Array">^\s*-</Rule>
    <Rule color="MapKey">\s*(?:&apos;.+?&apos;|&quot;.+?&quot;|\S+?)\s*(?=\:)</Rule>

  </RuleSet>

  <RuleSet name="Literals">
    <Import ruleSet="Strings" />
    <Rule color="Keyword">(?&lt;=\:|-)\s+(true|false|yes|no|on|off|null|NaN)\s*$</Rule>
    <Rule color="Number">(?&lt;=\:|-)\s*(\b0[xX][0-9a-fA-F]+|(\b\d+(\.[0-9]+)?|\.[0-9]+)([eE][+-]?[0-9]+)?)\s*$</Rule>
  </RuleSet>

  <RuleSet name="Strings">
    <Span color="String">
      <Begin>(?&lt;=\:|-)\s*&quot;</Begin>
      <End>&quot;</End>
      <RuleSet>
        <Span begin="\\" end="." foreground="IndianRed" />
      </RuleSet>
    </Span>
    <Span color="String">
      <Begin>(?&lt;=\:|-)\s*&apos;</Begin>
      <End>&apos;</End>
      <RuleSet>
        <Span begin="\\" end="." foreground="IndianRed" />
      </RuleSet>
    </Span>
  </RuleSet>

  <RuleSet name="JsonStrings">
    <Span color="Json">
      <Begin>&quot;</Begin>
      <End>&quot;</End>
      <RuleSet>
        <Span begin="\\" end="." foreground="IndianRed" />
      </RuleSet>
    </Span>
    <Span color="Json">
      <Begin>&apos;</Begin>
      <End>&apos;</End>
      <RuleSet>
        <Span begin="\\" end="." foreground="IndianRed" />
      </RuleSet>
    </Span>
  </RuleSet>

  <RuleSet name="JsonObject">
    <Import ruleSet="JsonExpression" />
    <Rule color="Punctuation">,</Rule>
  </RuleSet>

  <RuleSet name="JsonArray">
    <Import ruleSet="JsonExpression"/>
    <Rule color="Punctuation">,</Rule>
  </RuleSet>

  <RuleSet name="JsonExpression">
    <Span color="Comment" begin="#" />
    <Span color="Json" ruleSet="JsonObject" multiline="true">
      <Begin>\{</Begin>
      <End>\}</End>
    </Span>
    <Span color="Json" ruleSet="JsonArray" multiline="true">
      <Begin>\[</Begin>
      <End>\]</End>
    </Span>
    <Import ruleSet="JsonStrings" />
  </RuleSet>

</SyntaxDefinition>

Thanks, much appreciated! I suspect you're right that it might not be possible to properly define YAML with RegEx's (I'm not sure exactly how to build a regex for 'must match # of spaces in line above' and other YAML concepts...). If it helps, YAML was designed to be 'easy to read and write at the expense of being hard to parse' (as opposed to JSON where easy parsing was prioritized above ease of reading and writing), and I often see the YAML syntax highlighting in VS differ from how the YAML parser actually reads the file - so you're not the only one to struggle with it.

At the moment, I think anything is better than nothing. I'll try it out (probably be a little while before I can get to it) and let you know how it goes.