$ man 7 re_format produced this output

RE_FORMAT(7)       FreeBSD Miscellaneous Information Manual       RE_FORMAT(7)

NNAAMMEE
     rree__ffoorrmmaatt - POSIX 1003.2 regular expressions

DDEESSCCRRIIPPTTIIOONN
     Regular expressions ("REs"), as defined in IEEE Std 1003.2 ("POSIX.2"),
     come in two forms: modern REs (roughly those of egrep(1); 1003.2 calls
     these "extended" REs) and obsolete REs (roughly those of ed(1); 1003.2
     "basic" REs).  Obsolete REs mostly exist for backward compatibility in
     some old programs; they will be discussed at the end.  IEEE Std 1003.2
     ("POSIX.2") leaves some aspects of RE syntax and semantics open; `<**>'
     marks decisions on these aspects that may not be fully portable to other
     IEEE Std 1003.2 ("POSIX.2") implementations.

     A (modern) RE is one<**> or more non-empty<**> _b_r_a_n_c_h_e_s, separated by
     `|'.  It matches anything that matches one of the branches.

     A branch is one<**> or more _p_i_e_c_e_s, concatenated.  It matches a match for
     the first, followed by a match for the second, etc.

     A piece is an _a_t_o_m possibly followed by a single<**> `*', `+', `?', or
     _b_o_u_n_d.  An atom followed by `*' matches a sequence of 0 or more matches
     of the atom.  An atom followed by `+' matches a sequence of 1 or more
     matches of the atom.  An atom followed by `?' matches a sequence of 0 or
     1 matches of the atom.

     A _b_o_u_n_d is `{' followed by an unsigned decimal integer, possibly followed
     by `,' possibly followed by another unsigned decimal integer, always
     followed by `}'.  The integers must lie between 0 and RE_DUP_MAX
     (255<**>) inclusive, and if there are two of them, the first may not
     exceed the second.  An atom followed by a bound containing one integer _i
     and no comma matches a sequence of exactly _i matches of the atom.  An
     atom followed by a bound containing one integer _i and a comma matches a
     sequence of _i or more matches of the atom.  An atom followed by a bound
     containing two integers _i and _j matches a sequence of _i through _j
     (inclusive) matches of the atom.

     An atom is a regular expression enclosed in `()' (matching a match for
     the regular expression), an empty set of `()' (matching the null
     string)<**>, a _b_r_a_c_k_e_t _e_x_p_r_e_s_s_i_o_n (see below), `.' (matching any single
     character), `^' (matching the null string at the beginning of a line),
     `$' (matching the null string at the end of a line), a `\' followed by
     one of the characters `^.[$()|*+?{\' (matching that character taken as an
     ordinary character), a `\' followed by any other character<**> (matching
     that character taken as an ordinary character, as if the `\' had not been
     present<**>), or a single character with no other significance (matching
     that character).  A `{' followed by a character other than a digit is an
     ordinary character, not the beginning of a bound<**>.  It is illegal to
     end an RE with `\'.

     A _b_r_a_c_k_e_t _e_x_p_r_e_s_s_i_o_n is a list of characters enclosed in `[]'.  It
     normally matches any single character from the list (but see below).  If
     the list begins with `^', it matches any single character (but see below)
     _n_o_t from the rest of the list.  If two characters in the list are
     separated by `-', this is shorthand for the full _r_a_n_g_e of characters
     between those two (inclusive) in the collating sequence, e.g. `[0-9]' in
     ASCII matches any decimal digit.  It is illegal<**> for two ranges to
     share an endpoint, e.g. `a-c-e'.  Ranges are very collating-sequence-
     dependent, and portable programs should avoid relying on them.

     To include a literal `]' in the list, make it the first character
     (following a possible `^').  To include a literal `-', make it the first
     or last character, or the second endpoint of a range.  To use a literal
     `-' as the first endpoint of a range, enclose it in `[.' and `.]' to make
     it a collating element (see below).  With the exception of these and some
     combinations using `[' (see next paragraphs), all other special
     characters, including `\', lose their special significance within a
     bracket expression.

     Within a bracket expression, a collating element (a character, a multi-
     character sequence that collates as if it were a single character, or a
     collating-sequence name for either) enclosed in `[.' and `.]' stands for
     the sequence of characters of that collating element.  The sequence is a
     single element of the bracket expression's list.  A bracket expression
     containing a multi-character collating element can thus match more than
     one character, e.g. if the collating sequence includes a `ch' collating
     element, then the RE `[[.ch.]]*c' matches the first five characters of
     `chchcc'.

     Within a bracket expression, a collating element enclosed in `[=' and
     `=]' is an equivalence class, standing for the sequences of characters of
     all collating elements equivalent to that one, including itself.  (If
     there are no other equivalent collating elements, the treatment is as if
     the enclosing delimiters were `[.' and `.]'.) For example, if `x' and `y'
     are the members of an equivalence class, then `[[=x=]]', `[[=y=]]', and
     `[xy]' are all synonymous.  An equivalence class may not<**> be an
     endpoint of a range.

     Within a bracket expression, the name of a _c_h_a_r_a_c_t_e_r _c_l_a_s_s enclosed in
     `[:' and `:]' stands for the list of all characters belonging to that
     class.  Standard character class names are:

           _a_l_n_u_m    digit    punct
           _a_l_p_h_a    graph    space
           _b_l_a_n_k    lower    upper
           _c_n_t_r_l    print    xdigit

     These stand for the character classes defined in ctype(3).  A locale may
     provide others.  A character class may not be used as an endpoint of a
     range.

     A bracketed expression like `[[:class:]]' can be used to match a single
     character that belongs to a character class.  The reverse, matching any
     character that does not belong to a specific class, the negation operator
     of bracket expressions may be used: `[^[:class:]]'.

     There are two special cases<**> of bracket expressions: the bracket
     expressions `[[:<:]]' and `[[:>:]]' match the null string at the
     beginning and end of a word respectively.  A word is defined as a
     sequence of word characters which is neither preceded nor followed by
     word characters.  A word character is an _a_l_n_u_m character (as defined by
     ctype(3)) or an underscore.  This is an extension, compatible with but
     not specified by IEEE Std 1003.2 ("POSIX.2"), and should be used with
     caution in software intended to be portable to other systems.  The
     additional word delimiters `\<' and `\>' are provided to ease
     compatibility with traditional SVR4 systems but are not portable and
     should be avoided.

     In the event that an RE could match more than one substring of a given
     string, the RE matches the one starting earliest in the string.  If the
     RE could match more than one substring starting at that point, it matches
     the longest.  Subexpressions also match the longest possible substrings,
     subject to the constraint that the whole match be as long as possible,
     with subexpressions starting earlier in the RE taking priority over ones
     starting later.  Note that higher-level subexpressions thus take priority
     over their lower-level component subexpressions.

     Match lengths are measured in characters, not collating elements.  A null
     string is considered longer than no match at all.  For example, `bb*'
     matches the three middle characters of `abbbc',
     `(wee|week)(knights|nights)' matches all ten characters of `weeknights',
     when `(.*).*' is matched against `abc' the parenthesized subexpression
     matches all three characters, and when `(a*)*' is matched against `bc'
     both the whole RE and the parenthesized subexpression match the null
     string.

     If case-independent matching is specified, the effect is much as if all
     case distinctions had vanished from the alphabet.  When an alphabetic
     that exists in multiple cases appears as an ordinary character outside a
     bracket expression, it is effectively transformed into a bracket
     expression containing both cases, e.g. `x' becomes `[xX]'.  When it
     appears inside a bracket expression, all case counterparts of it are
     added to the bracket expression, so that (e.g.)  `[x]' becomes `[xX]' and
     `[^x]' becomes `[^xX]'.

     No particular limit is imposed on the length of REs<**>.  Programs
     intended to be portable should not employ REs longer than 256 bytes, as
     an implementation can refuse to accept such REs and remain POSIX-
     compliant.

     Obsolete ("basic") regular expressions differ in several respects.  `|'
     is an ordinary character and there is no equivalent for its
     functionality.  `+' and `?' are ordinary characters, and their
     functionality can be expressed using bounds (`{1,}' or `{0,1}'
     respectively).  Also note that `x+' in modern REs is equivalent to `xx*'.
     The delimiters for bounds are `\{' and `\}', with `{' and `}' by
     themselves ordinary characters.  The parentheses for nested
     subexpressions are `\(' and `\)', with `(' and `)' by themselves ordinary
     characters.  `^' is an ordinary character except at the beginning of the
     RE or<**> the beginning of a parenthesized subexpression, `$' is an
     ordinary character except at the end of the RE or<**> the end of a
     parenthesized subexpression, and `*' is an ordinary character if it
     appears at the beginning of the RE or the beginning of a parenthesized
     subexpression (after a possible leading `^').  Finally, there is one new
     type of atom, a _b_a_c_k _r_e_f_e_r_e_n_c_e: `\' followed by a non-zero decimal digit
     _d matches the same sequence of characters matched by the _dth
     parenthesized subexpression (numbering subexpressions by the positions of
     their opening parentheses, left to right), so that (e.g.)  `\([bc]\)\1'
     matches `bb' or `cc' but not `bc'.

SSEEEE AALLSSOO
     regex(3)

     _R_e_g_u_l_a_r _E_x_p_r_e_s_s_i_o_n _N_o_t_a_t_i_o_n, IEEE Std, 1003.2, section 2.8.

BBUUGGSS
     Having two kinds of REs is a botch.

     The current IEEE Std 1003.2 ("POSIX.2") spec says that `)' is an ordinary
     character in the absence of an unmatched `('; this was an unintentional
     result of a wording error, and change is likely.  Avoid relying on it.

     Back references are a dreadful botch, posing major problems for efficient
     implementations.  They are also somewhat vaguely defined (does
     `a\(\(b\)*\2\)*d' match `abbbd'?).  Avoid using them.

     IEEE Std 1003.2 ("POSIX.2") specification of case-independent matching is
     vague.  The "one case implies all cases" definition given above is
     current consensus among implementors as to the right interpretation.

     The syntax for word boundaries is incredibly ugly.

FreeBSD 12.0-RELEASE             June 30, 2014            FreeBSD 12.0-RELEASE