Handling HTML Markup with Drupal’s Migrate API

Benji Fisher

January 31, 2020

Introduction

About me

Benji Fisher
Hook 42
drupal.org: benjifisher
twitter: @benji17fisher

Follow along

Find a link to this presentation on my GitLab Pages:

Drupal 8 Migrate API

  • Upgrade Drupal 6 and Drupal 7 sites
  • Migrate sites from other systems to Drupal
  • Imports from external systems (feeds)

A robust, flexible tool.

Migrate API: structured data

  • file attachments
  • related taxonomy terms
  • references to authors
  • references to other nodes

Migrate API: unstructured text

What about unstructured text with HTML Markup?

  • Regular expressions (old)
  • HTML parsing (recent)

Our approach: we wrote new Migrate process plugins in Migrate Plus for Pega Systems/Isovera.

Outline

  • Introduction
  • Parsing HTML: regexp
  • Parsing HTML: DOMDocument
  • Drupal 8 Migrate API
  • (Possible) Future
  • Conclusion

Parsing HTML: regexp

At a glance

HTML

Regular expression

HTML or extract strings

Simple example (?)

Extract the URL from

Parsing HTML: preg_match()

Extract the URL:

Parsing HTML: not so simple

Complications:

  • HTML tags: match a or A
  • Other attributes: class, id, name, …
  • Single quotes or double quotes
  • Newlines within the HTML element
  • Are escaped quotes (like \") allowed in a URL?

Trick question: do not reinvent the wheel!

Parsing HTML: examples

Complications:

  • <a href="https://www.drupal.org">Drupal home page</a> (original)
  • <A href="https://www.drupal.org">Drupal home page</A>
  • <a class="ext-link" href=...
  • <a href='https://www.drupal.org'>Drupal home page</a>
  • <a href="https://www.dr\"upal.org">Drupal home page</a>

Parsing HTML: right answer, wrong question

Parsing HTML: innocent question

From StackOverflow:

I need to match all of these opening tags:

But not these:

Parsing HTML: Cthulhu (1/3)

The answer:

You can’t parse [X]HTML with regex. Because HTML can’t be parsed by regex. Regex is not a tool that can be used to correctly parse HTML. …

Parsing HTML: Cthulhu (2/3)

The answer:

Every time you attempt to parse HTML with regular expressions, the unholy child weeps the blood of virgins, and Russian hackers pwn your webapp. Parsing HTML with regex summons tainted souls into the realm of the living. …

Parsing HTML: Cthulhu (3/3)

The answer:

Have you tried using an XML parser instead?

Parsing HTML: DOM

At a glance

HTML

Document Object Model (DOM)

HTML or extract strings

DOMDocument basics

The DOM extension uses GNOME’s libxml library in the background. DOM includes XML Path Language (XPath) traversing.

Using Drupal’s Html Utility Class

XPath Examples

With $xpath->query($selector), …

$selector Matches
//a all <a> elements
//a[class="external"] all <a> elements with class="external"
//li[class="nav"]/a all <a> elements direct children of <li class="nav">

XPath Example (Complicated)

ANEDS/ANED[
  @s:id = ../ANEDOA[
    nc:OR/@s:ref = "ORG0"
  ]/CDR/@s:ref
]/NED/NEDQ[
  ../NEDRC/text() = "ExpeditedDenial"
]

From usdoj/foia-api on GitHub (whitespace added, tags abbreviated)

DOMDocument output

After processing, return an HTML string:

Drupal 8 Migrate API

ETL paradigm

In Drupal 8, the Migrate API follows the standard Extract, Transform, Load (ETL) structure:

  • Extract (source plugin): read data from the source
  • Transform (process plugins): change data to match the site’s structure
  • Load (destination plugin): save the data

The Transform/process phase is the right place to handle HTML processing.

At a glance

HTML

Migrate dom* process plugins

HTML

New process plugins for managing HTML

Four process plugins in the Migrate Plus module:

  • dom
  • dom_str_replace
  • dom_migration_lookup
  • dom_apply_styles

Goal: make it easy to process text fields with proper HTML parsing.

The dom plugin

  • Create DOMDocument object from string
  • Create string from DOMDocument object

dom_str_replace plugin

Change the subdomain during migration:

Use str_replace() or preg_replace() on the href attribute.

dom_apply_styles plugin

Search for an XPath expression. Replace with styles configured in the Editor module.

dom_migration_lookup

Like core Migrate’s migration_lookup plugin.

(Possible) Future

More process plugins

Different parsers than DOM

Just an FYI, my goto for HTML parsing has been querypath, it’s especially good if you’re dealing with old-school HTML (no </p>, etc.). ‒ mikeryan on #2958281-7

Different parsers than DOM

  • url source plugin data parsers
  • Make process plugins data extensible: use core typed data.

HTML5

Masterminds\HTML5::loadHTML() -> \DOMDocument

Conclusion

References

Questions

Copyleft

Creative Commons License
This slide deck by Benji Fisher is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.
Based on a work at https://gitlab.com/benjifisher/slide-decks.