Invalid link-external-filter

The link-external-filter option is not a regular expression Quarto can compile, so it is ignored.

Q-13-14 — Invalid link-external-filter

The link-external-filter option is not a regular expression Quarto can compile, so it is ignored.

What this means

link-external-filter tells Quarto which absolute links count as internal when link-external-icon or link-external-newwindow is on. Any link whose URL matches the expression is left alone.

format:
  html:
    link-external-icon: true
    link-external-filter: '^https://(www\.)?quarto\.org/'

This warning fires when the expression does not compile. The render still succeeds. Quarto ignores the filter and falls back to the default, which treats links under website.site-url as internal. Without a site-url, every absolute http(s) link counts as external.

Why this happens

• The pattern uses JavaScript-only syntax. Quarto 1 evaluated this filter as a JavaScript regular expression in the browser. Quarto 2 compiles it with Rust’s regex syntax, which has no look-around ((?=…), (?!…), (?<=…), (?<!…)) and no backreferences (\1).
• A typo. An unbalanced parenthesis or bracket, or a dangling \ at the end of the pattern.

How to fix

• Rewrite look-arounds as a plain prefix match. For example, to treat both quarto.org and posit.co as internal, list them instead of excluding everything else:

  # JavaScript-only (look-ahead): rejected
  link-external-filter: '^https://(?!example\.com)'
  # Rust regex: accepted
  link-external-filter: '^https://(quarto\.org|posit\.co)/'

• Quote the pattern in single quotes in YAML so that backslashes reach Quarto unchanged.
• Escaping / as \/ is harmless. Quarto 1’s documentation example, ^(?:http:|https:)\/\/www\.quarto\.org\/custom, still works.

Related

• Q-13-4 — body link referencing a missing document.