It doesn't appear to be parsing at all. It's just looking for patterns.

If you look at languages.json in RulesEngine/Resources, files with the extension .html (and some others) are recognized as "html", with type "code":

  {
    "name": "html",
    "extensions": [ ".html", ".htm", ".cshtml", ".tmpl" ],
    "type": "code"
  },

This sets the scope for the patterns in AppInspector/rules/default; e.g.,

  {
    "name": "Content Management Framework: Wordpress",
    "id": "AI021200",
    "description": "Development Framework: Wordpress",
    "applies_to": [ "javascript", "html" ],
    "tags": [ "Framework.CMS.Wordpress" ],
    "severity": "moderate",
    "patterns": [
      {
        "pattern": "wordpress",
        "type": "string",
        "scopes": [ "code", "comment" ],
        "modifiers": [ "i" ],
        "confidence": "high"
      }
    ]
  },

This seems like it would be prone to a lot of false positives, but I haven't tried the tool.

Boy, are you in for a fun StackExchange read: https://stackoverflow.com/a/1732454 .

You can't parse it properly with a Regular Expression, but you can parse it with regex-like systems. However I doubt it is parsing outright - it only has to look for certain keywords and patterns that indicate certain behaviors.

Correct. We don't need to parse it per se just look for use of features that are easy to identify like XmlHttpRequest, Json.Parse use etc.