Tighten regex a bit.

If we need to make this more robust in the future, we can revert this as needed.

site/layouts/shortcodes/example.html

@@ -19,7 +19,7 @@
 {{- end -}}
 
 {{- if eq $show_markup true -}}
-  {{- $content := replaceRE `<svg.*class=.bd\-placeholder\-img(?:\-lg)?(?:\s*?bd\-placeholder\-img\-lg)?\s?(.*?)".*?<\/svg>\n` `<img src="..." class="$1" alt="...">` $input -}}
-  {{- $content = replaceRE `(class="\s*?")` "" $content -}}
+  {{- $content := replaceRE `<svg class="bd\-placeholder\-img(?:\-lg)?(?: *?bd\-placeholder\-img\-lg)? ?(.*?)".*?<\/svg>\n` `<img src="..." class="$1" alt="...">` $input -}}
+  {{- $content = replaceRE `(class=" *?")` "" $content -}}
   {{- highlight (trim $content "\n") "html" "" -}}
 {{- end -}}