re2c uses the following syntax for regular expressions:

*    ``"foo"`` case-sensitive string literal

*    ``'foo'`` case-insensitive string literal

*    ``[a-xyz]``, ``[^a-xyz]`` character class (possibly negated)

*    ``.`` any character except newline

*    ``R \ S`` difference of character classes ``R`` and ``S``

*    ``R*`` zero or more occurrences of ``R``

*    ``R+`` one or more occurrences of ``R``

*    ``R?`` optional ``R``

*    ``R{n}`` repetition of ``R`` exactly ``n`` times

*    ``R{n,}`` repetition of ``R`` at least ``n`` times

*    ``R{n,m}`` repetition of ``R`` from ``n`` to ``m`` times

*    ``(R)`` just ``R``; parentheses are used to override precedence or for POSIX-style submatch

*    ``R S`` concatenation: ``R`` followed by ``S``

*    ``R | S`` alternative: ``R or S``

*    ``R / S`` lookahead: ``R`` followed by ``S``, but ``S`` is not consumed

*    ``name`` the regular expression defined as ``name`` (or literal string ``"name"`` in Flex compatibility mode)

*    ``{name}`` the regular expression defined as ``name`` in Flex compatibility mode

*    ``@stag`` an *s-tag*: saves the last input position at which ``@stag`` matches in a variable named ``stag``

*    ``#mtag`` an *m-tag*: saves all input positions at which ``#mtag`` matches in a variable named ``mtag``

Character classes and string literals may contain the following escape sequences:
``\a``, ``\b``, ``\f``, ``\n``, ``\r``, ``\t``, ``\v``, ``\\``, octal escapes ``\ooo`` and hexadecimal escapes ``\xhh``, ``\uhhhh`` and ``\Uhhhhhhhh``.