The complete list of extensions to MySQL regexes that I included is:

    '\w' => '[_[:alnum:]]',
    '\W' => '[^_[:alnum:]]',
    '\s' => '[[:space:]]',
    '\S' => '[^[:space:]]',
    '\d' => '[0-9]',
    '\D' => '[^0-9]',
    '\n' => "\n",
    '\t' => "\t",
    '\b' => '[[:<:]]',
    '\B' => '[[:>:]]',
    '\A' => '^',
    '\z' => '$',
    '\Z' => "\n?\$",
    '^'  => "(^|\n)",
    '$'  => "(\n|\$)",
    \xHH
    \0OOO (probably should be \[0123]OO instead, *sigh*)
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Plus you can get this error:

We don't allow a () that contains +, *, or {} to have +, *, or {} applied to it (to prevent excess server load).