Narrow url-ignore for lightbox links (#147)

After #132 @angelplasma
proposed setting the lightbox links to href="#_" which they now all are.

In order to not ignore all links containing "#", this commit now regex
matches the complete string by using begin-of-string and end-of-string
anchors.

Additionally, this uses the comma separated list of regular expressions,
as recommended in html-proofer's README.

script/test.sh

@@ -4,11 +4,15 @@ set -e # halt script on error
 # Build the site
 bundle exec jekyll build
 
-# Check for broken links and missing alt tags, ignore edit links to GitHub as they might not exist yet
+# Check for broken links and missing alt tags, ignoring all links matching
+# the regular expression patterns:
+#  containing "github.com" as they might not exist yet
+#  containing "twitter.com" because twitter is broken
+#  exactly "#_" (anchored by begin of line and end of line)
+# See also:
+#  https://ruby-doc.org/core-2.7.1/Regexp.html
+#  https://github.com/gjtorikian/html-proofer/blob/main/README.md
 
 bundle exec htmlproofer \
-    --url-ignore "/github.com/(.*)/edit/" \
-    --url-ignore "/github.com/,/twitter.com/" \
-    --url-ignore "/#/" \
+    --url-ignore '/github\.com/,/twitter\.com/,/^#_$/' \
     ./_site
-