RE_FORMAT(7)               OpenBSD Reference Manual               RE_FORMAT(7)

NAME
     re_format - POSIX regular expressions

DESCRIPTION
     Regular expressions (REs), as defined in IEEE Std 1003.1-2004
     (``POSIX''), come in two forms: basic regular expressions (BREs) and ex-
     tended regular expressions (EREs).  Both forms of regular expressions are
     supported by the interfaces described in regex(3).  Applications dealing
     with regular expressions may use one or the other form (or indeed both).
     For example, ed(1) uses BREs, whilst egrep(1) talks EREs.  Consult the
     manual page for the specific application to find out which it uses.

     POSIX leaves some aspects of RE syntax and semantics open; `**' marks de-
     cisions on these aspects that may not be fully portable to other POSIX
     implementations.

     This manual page first describes regular expressions in general, specifi-
     cally extended regular expressions, and then discusses differences be-
     tween them and basic regular expressions.

EXTENDED REGULAR EXPRESSIONS
     An ERE is one** or more non-empty** branches, separated by `|'.  It
     matches anything that matches one of the branches.

     A branch is one** or more pieces, concatenated.  It matches a match for
     the first, followed by a match for the second, etc.

     A piece is an atom possibly followed by a single** `*', `+', `?', or
     bound.  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 bound is `{' followed by an unsigned decimal integer, possibly followed
     by `,' possibly followed by another unsigned decimal integer, always fol-
     lowed by `}'.  The integers must lie between 0 and RE_DUP_MAX (255**) in-
     clusive, and if there are two of them, the first may not exceed the sec-
     ond.  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 in-
     tegers 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 part of the
     regular expression), an empty set of `()' (matching the null string)**, a
     bracket expression (see below), `.' (matching any single character), `^'
     (matching the null string at the beginning of a line), `$' (matching the
     null string at the end of a line), a `\' followed by one of the charac-
     ters `^.[$()|*+?{\' (matching that character taken as an ordinary charac-
     ter), 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 charac-
     ter, not the beginning of a bound**.  It is illegal to end an RE with
     `\'.

     A bracket expression is a list of characters enclosed in `[]'.  It nor-
     mally matches any single character from the list (but see below).  If the
     list begins with `^', it matches any single character not from the rest
     of the list (but see below).  If two characters in the list are separated
     by `-', this is shorthand for the full range 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 (fol-
     lowing 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 charac-
     ters, including `\', lose their special significance within a bracket ex-
     pression.

     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 character class enclosed in
     `[:' and `:]' stands for the list of all characters belonging to that
     class.  Standard character class names are:

           alnum   digit   punct
           alpha   graph   space
           blank   lower   upper
           cntrl   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.

     There are two special cases** of bracket expressions: the bracket expres-
     sions `[[:<:]]' and `[[:>:]]' match the null string at the beginning and
     end of a word, respectively.  A word is defined as a sequence of charac-
     ters starting and ending with a word character which is neither preceded
     nor followed by word characters.  A word character is an alnum character
     (as defined by ctype(3)) or an underscore.  This is an extension, compat-
     ible with but not specified by POSIX, and should be used with caution in
     software intended to be portable to other systems.

     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 expres-
     sion containing both cases, e.g. `x' becomes `[xX]'.  When it appears in-
     side a bracket expression, all case counterparts of it are added to the
     bracket expression, so that, for example, `[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 imple-
     mentation can refuse to accept such REs and remain POSIX-compliant.

     The following is a list of extended regular expressions:

     c       Any character c not listed below matches itself.

     \c      Any backslash-escaped character c matches itself.

     .       Matches any single character that is not a newline (`\n').

     [char-class]
             Matches any single character in char-class.  To include a `]' in
             char-class, it must be the first character.  A range of charac-
             ters may be specified by separating the end characters of the
             range with a `-'; e.g. a-z specifies the lower case characters.
             The following literal expressions can also be used in char-class
             to specify sets of characters:

                   [:alnum:] [:cntrl:] [:lower:] [:space:]
                   [:alpha:] [:digit:] [:print:] [:upper:]
                   [:blank:] [:graph:] [:punct:] [:xdigit:]

             If `-' appears as the first or last character of char-class, then
             it matches itself.  All other characters in char-class match
             themselves.

             Patterns in char-class of the form [.col-elm.] or [=col-elm=],
             where col-elm is a collating element, are interpreted according
             to setlocale(3) (not currently supported).

     [^char-class]
             Matches any single character, other than newline, not in char-
             class.  char-class is defined as above.

     ^       If `^' is the first character of a regular expression, then it
             anchors the regular expression to the beginning of a line.  Oth-
             erwise, it matches itself.

     $       If `$' is the last character of a regular expression, it anchors
             the regular expression to the end of a line.  Otherwise, it
             matches itself.

     [[:<:]]
             Anchors the single character regular expression or subexpression
             immediately following it to the beginning of a word.

     [[:>:]]
             Anchors the single character regular expression or subexpression
             immediately following it to the end of a word.

     (re)    Defines a subexpression re.  Any set of characters enclosed in
             parentheses matches whatever the set of characters without paren-
             theses matches (that is a long-winded way of saying the con-
             structs `(re)' and `re' match identically).

     *       Matches the single character regular expression or subexpression
             immediately preceding it zero or more times.  If `*' is the first
             character of a regular expression or subexpression, then it
             matches itself.  The `*' operator sometimes yields unexpected re-
             sults.  For example, the regular expression b* matches the begin-
             ning of the string "abbb" (as opposed to the substring "bbb"),
             since a null match is the only leftmost match.

     +       Matches the singular character regular expression or subexpres-
             sion immediately preceding it one or more times.

     ?       Matches the singular character regular expression or subexpres-
             sion immediately preceding it 0 or 1 times.

     {n,m} {n,} {n}
             Matches the single character regular expression or subexpression
             immediately preceding it at least n and at most m times.  If m is
             omitted, then it matches at least n times.  If the comma is also
             omitted, then it matches exactly n times.

     |       Used to separate patterns.  For example, the pattern `cat|dog'
             matches either `cat' or `dog'.

BASIC REGULAR EXPRESSIONS
     Basic regular expressions differ in several respects:

        o   `|', `+', and `?' are ordinary characters and there is no equiva-
            lent for their functionality.

        o   The delimiters for bounds are `\{' and `\}', with `{' and `}' by
            themselves ordinary characters.

        o   The parentheses for nested subexpressions are `\(' and `\)', with
            `(' and `)' by themselves ordinary characters.

        o   `^' is an ordinary character except at the beginning of the RE
            or** the beginning of a parenthesized subexpression.

        o   `$' is an ordinary character except at the end of the RE or** the
            end of a parenthesized subexpression.

        o   `*' is an ordinary character if it appears at the beginning of the
            RE or the beginning of a parenthesized subexpression (after a pos-
            sible leading `^').

        o   Finally, there is one new type of atom, a back-reference: `\' fol-
            lowed by a non-zero decimal digit d matches the same sequence of
            characters matched by the dth parenthesized subexpression (number-
            ing subexpressions by the positions of their opening parentheses,
            left to right), so that, for example, `\([bc]\)\1' matches `bb' or
            `cc' but not `bc'.

     The following is a list of basic regular expressions:

     c       Any character c not listed below matches itself.

     \c      Any backslash-escaped character c, except for `{', `}', `(', and
             `)', matches itself.

     .       Matches any single character that is not a newline (`\n').

     [char-class]
             Matches any single character in char-class.  To include a `]' in
             char-class, it must be the first character.  A range of charac-
             ters may be specified by separating the end characters of the
             range with a `-'; e.g. a-z specifies the lower case characters.
             The following literal expressions can also be used in char-class
             to specify sets of characters:

                   [:alnum:] [:cntrl:] [:lower:] [:space:]
                   [:alpha:] [:digit:] [:print:] [:upper:]
                   [:blank:] [:graph:] [:punct:] [:xdigit:]

             If `-' appears as the first or last character of char-class, then
             it matches itself.  All other characters in char-class match
             themselves.

             Patterns in char-class of the form [.col-elm.] or [=col-elm=],
             where col-elm is a collating element, are interpreted according
             to setlocale(3) (not currently supported).

     [^char-class]
             Matches any single character, other than newline, not in char-
             class.  char-class is defined as above.

     ^       If `^' is the first character of a regular expression, then it
             anchors the regular expression to the beginning of a line.  Oth-
             erwise, it matches itself.

     $       If `$' is the last character of a regular expression, it anchors
             the regular expression to the end of a line.  Otherwise, it
             matches itself.

     [[:<:]]
             Anchors the single character regular expression or subexpression
             immediately following it to the beginning of a word.

     [[:>:]]
             Anchors the single character regular expression or subexpression
             immediately following it to the end of a word.

     \(re\)  Defines a subexpression re.  Subexpressions may be nested.  A
             subsequent backreference of the form \n, where n is a number in
             the range [1,9], expands to the text matched by the nth subex-
             pression.  For example, the regular expression \(.*\)\1 matches
             any string consisting of identical adjacent substrings.  Subex-
             pressions are ordered relative to their left delimiter.

     *       Matches the single character regular expression or subexpression
             immediately preceding it zero or more times.  If `*' is the first
             character of a regular expression or subexpression, then it
             matches itself.  The `*' operator sometimes yields unexpected re-
             sults.  For example, the regular expression b* matches the begin-
             ning of the string "abbb" (as opposed to the substring "bbb"),
             since a null match is the only leftmost match.

     \{n,m\} \{n,\} \{n\}
             Matches the single character regular expression or subexpression
             immediately preceding it at least n and at most m times.  If m is
             omitted, then it matches at least n times.  If the comma is also
             omitted, then it matches exactly n times.

SEE ALSO
     ctype(3), regex(3)

STANDARDS
     IEEE Std 1003.1-2004 (``POSIX''): Base Definitions, Chapter 9 (Regular
     Expressions).

BUGS
     Having two kinds of REs is a botch.

     The current POSIX spec says that `)' is an ordinary character in the ab-
     sence 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.

     POSIX's specification of case-independent matching is vague.  The ``one
     case implies all cases'' definition given above is the current consensus
     among implementors as to the right interpretation.

     The syntax for word boundaries is incredibly ugly.

OpenBSD 4.7                      May 31, 2007                                6
