Apache SPA mod_rewrite 構成が正規表現でドットを使用し、キャレットが Apache をクラッシュさせるのはなぜですか?

Question

mod_rewrite に基づく一般的な Apache SPA 構成:

DocumentRoot "${webDir}"

<Directory "${webDir}">
  LogLevel rewrite:trace8

  RewriteEngine On
  RewriteCond %{SCRIPT_FILENAME} !-f
  RewriteRule . index.html [L]
</Directory>

最後のルールの正規表現をキャレットに置き換えた場合:

  RewriteRule ^ index.html [L]

Apache がセグメント違反を起こしますhttp://localhost/(空でないパスであれば動作します)。エラー ログは疑わしいものではありません:

[rewrite:trace3] mod_rewrite.c(493): strip per-dir prefix: /home/user/work/out/ -> 
[rewrite:trace3] mod_rewrite.c(493): applying pattern '^' to uri ''
[rewrite:trace4] mod_rewrite.c(493): RewriteCond: input='/home/user/work/out/' pattern='!-f' => matched
[rewrite:trace2] mod_rewrite.c(493): rewrite '' -> 'index.html'
[rewrite:trace3] mod_rewrite.c(493): add per-dir prefix: index.html -> /home/user/work/out/index.html
[rewrite:trace2] mod_rewrite.c(493): strip document_root prefix: /home/user/work/out/index.html -> /index.html
[rewrite:trace1] mod_rewrite.c(493): internal redirect with /index.html [INTERNAL REDIRECT]

ドット付きのログ（クラッシュにはならない）は次のとおりです。

[rewrite:trace3] mod_rewrite.c(493) strip per-dir prefix: /home/user/work/out/ -> 
[rewrite:trace3] mod_rewrite.c(493) applying pattern '.' to uri ''
[rewrite:trace1] mod_rewrite.c(493) pass through /home/user/work/out/
[rewrite:trace3] mod_rewrite.c(493) strip per-dir prefix: /home/user/work/out/index.html -> index.html
[rewrite:trace3] mod_rewrite.c(493) applying pattern '.' to uri 'index.html'
[rewrite:trace4] mod_rewrite.c(493) RewriteCond: input='/home/user/work/out/index.html' pattern='!-f' => not-matched
[rewrite:trace1] mod_rewrite.c(493) pass through /home/user/work/out/index.html