# Matches an HTML ID attribute

(?<=             # We only want to match an ID if it's in a tag
                 # but we don't want to match the tag itself, so we use a lookbehind
    <[\w-]+      # to find the tag
    [^>]+?(?=    # and anything but closing carets
        \z|id    # until the end of the string or 'id'
    )
)
id               # now we match the id itself
\s{0,}           # and optional whitespace
=                # and the equal
\s{0,}           # and more optional whitespace
(?>              # Then, there are 3 ways we can have an ID
    '            # Between single quotes
    (?<ID>       # The ID is...
        ((?:
            ''|  # any double paired quote OR
            \\'| # any slash-escaped single quote
            [^'] # any non-quote
        )*)
    )
    '            # followed by the closing quote
    |            # OR
    "            # double quotes
    (?<ID>       # The ID is...
    .*?          # anything until
    (?<!(\\))    # a not-escape
    )"           # double-quote
    |            # OR
    (?<ID>       # The ID is..
        [\w-]+   # any number of word characters or dashes.
    )
)