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re_format(7)         BSD Miscellaneous Information Manual         re_format(7)


NAME

     re_format -- POSIX 1003.2 regular expressions


DESCRIPTION

     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 seman-
     tics 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= 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=)
     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 fol-
     lowed 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 bracket expression (see below), `.' (matching any single
     character), `^' (matching the null string at the beginning of a line),
     `$' (matching the null string at the end of a line), a `\' followed by
     one of the characters `^.[$()|*+?{\' (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 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 (but see below) not
     from the rest of the list.  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
     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 end-
     point 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.

     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 expres-
     sions `[[:<:]]' 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 alnum 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 soft-
     ware 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
     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 imple-
     mentation 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 functional-
     ity.  `+' 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 subex-
     pression, `$' is an ordinary character except at the end of the RE or=
     the end of a parenthesized subexpression, and `*' is an ordinary charac-
     ter if it appears at the beginning of the RE or the beginning of a paren-
     thesized subexpression (after a possible leading `^').  Finally, there is
     one new type of atom, a back reference: `\' followed by a non-zero deci-
     mal 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'.


ENHANCED FEATURES

     When the REG_ENHANCED flag is passed to one of the regcomp() variants,
     additional features are activated.  Like the enhanced regex implementa-
     tions in scripting languages such as perl(1) and python(1), these addi-
     tional features may conflict with the IEEE Std 1003.2 (``POSIX.2'') stan-
     dards in some ways.  Use this with care in situations which require
     portability (including to past versions of the Mac OS X using the previ-
     ous regex implementation).

     For enhanced basic REs, `+', `?' and `|' remain regular characters, but
     `\+', `\?' and `\|' have the same special meaning as the unescaped char-
     acters do for extended REs, i.e., one or more matches, zero or one
     matches and alteration, respectively.  For enhanced extended REs, back
     references are available.  Additional enhanced features are listed below.

     Within a bracket expression, most characters lose their magic.  This also
     applies to the additional enhanced features, which don't operate inside a
     bracket expression.

   Assertions (available for both enhanced basic and enhanced extended REs)
     In addition to `^' and `$' (the assertions that match the null string at
     the beginning and end of line, respectively), the following assertions
     become available:

           \<  Matches the null string at the beginning of a word.  This is
               equivalent to `[[:<:]]'.

           \>  Matches the null string at the end of a word.  This is equiva-
               lent to `[[:>:]]'.

           \b  Matches the null string at a word boundary (either the begin-
               ning or end of a word).

           \B  Matches the null string where there is no word boundary.  This
               is the opposite of `\b'.

   Shortcuts (available for both enhanced basic and enhanced extended REs)
     The following shortcuts can be used to replace more complicated bracket
     expressions.

           \d  Matches a digit character.  This is equivalent to
               `[[:digit:]]'.

           \D  Matches a non-digit character.  This is equivalent to
               `[^[:digit:]]'.

           \s  Matches a space character.  This is equivalent to
               `[[:space:]]'.

           \S  Matches a non-space character.  This is equivalent to
               `[^[:space:]]'.

           \w  Matches a word character.  This is equivalent to
               `[[:alnum:]_]'.

           \W  Matches a non-word character.  This is equivalent to
               `[^[:alnum:]_]'.

   Literal Sequences (available for both enhanced basic and enhanced extended
     REs)
     Literals are normally just ordinary characters that are matched directly.
     Under enhanced mode, certain character sequences are converted to spe-
     cific literals.

           \a  The ``bell'' character (ASCII code 7).

           \e  The ``escape'' character (ASCII code 27).

           \f  The ``form-feed'' character (ASCII code 12).

           \n  The ``new-line/line-feed'' character (ASCII code 10).

           \r  The ``carriage-return'' character (ASCII code 13).

           \t  The ``horizontal-tab'' character (ASCII code 9).

     Literals can also be specified directly, using their wide character val-
     ues.  Note that when matching a multibyte character string, the string's
     bytes are converted to wide character before comparing.  This means that
     a single literal wide character value may match more than one string
     byte, depending on the locale's wide character encoding.

           \xx..    An arbitray eight-bit value.  The x.. sequence represents
                    zero, one or two hexadecimal digits.  (Note: if x.. is
                    less than two hexadecimal digits, and the character fol-
                    lowing this sequence happens to be a hexadecimal digit,
                    use the (following) brace form to avoid confusion.)

           \x{x..}  An arbitrary, up to 32-bit value.  The x.. sequence is an
                    arbitrary sequence of hexadecimal digits that is long
                    enough to represent the necessary value.

   Inline Literal Mode (available for both enhanced basic and enhanced
     extended REs)
     A `\Q' sequence causes literal (``quote'') mode to be entered, while `\E'
     ends literal mode, and returns to normal regular expression processing.
     This is similar to specifying the REG_NOSPEC (or REG_LITERAL) option to
     regcomp(), except that rather than applying to the whole RE string, it
     only applies to the part between the `\Q' and `\E'.  Note that it is not
     possible to have a `\E' in the middle of an inline literal range, as that
     would terminate literal mode prematurely.

   Minimal Repetitions (available for enhanced extended REs only)
     By default, the repetition operators, `*', bound, `?' and `+' are greedy;
     they try to match as many times as possible.  In enhanced mode, appending
     a `?' to a repetition operator makes it minimal (or ungreedy); it tries
     to match the fewest number of times (including zero times, as appropri-
     ate).

     For example, against the string `aaa', the RE `a*' would match the entire
     string, while `a*?' would match the null string at the beginning of the
     line (matches zero times).  Likewise, against the string `ababab', the RE
     `.*b', would also match the entire string, while `.*?b' would only match
     the first two characters.

     The regcomp() flag REG_UNGREEDY will make the regular (greedy) repetition
     operators ungreedy by default.  Appending `?' makes them greedy again.

     Note that minimal repetitions are not specified by an official standard,
     so there may be differences between different implementations.  In the
     current implementation, minimal repetitions have a high precedence, and
     can cause other standards requirements to be violated.  For instance, on
     the string `aaaaa', the RE `(aaa??)*' will only match the first four
     characters, violating the rules that the longest possible match is made
     and the longest subexpressions are matched.  Using `(aaa??)*$' forces the
     entire string to be matched.

   Non-capturing Parenthesized Subexpressions (available for enhanced extended
     REs only)
     Normally, the match offsets to parenthesized subexpressions are recorded
     in the pmatch array (that is, when REG_NOSUB is not specified, and nmatch
     is large enough to encompass the parenthesized subexpression in ques-
     tion).  In enhanced mode, if the first two characters following the left
     parenthesis are `?:', grouping of the remaining contents is done, but the
     corresponding offsets are not recorded in the pmatch array.  For example,
     against the string `fubar', the RE `(fu)(bar)' would have two subexpres-
     sion matches in pmatch; the first for `fu' and the second for `bar'.  But
     with the RE `(?:fu)(bar)', there would only be one subexpression match,
     that of `bar'.  Furthermore, against the string `fufubar', the RE
     `(?fu)*(bar)' would again match the entire string, but only `bar' would
     be recorded in pmatch.

   Inline Options (available for enhanced extended REs only)
     Like the inline literal mode mentioned above, other options can be
     switched on and off for part of a RE.  `(?o..)' will turn on the options
     specified in o.. (one or more options characters; see below), while
     `(?-o..)' will turn off the specified options, and `(?o1..-o2..)' will
     turn on the first set of options, and turn off the second set.

     The available options are:

           i  Turning on this option will ignore case during matching, while
              turning off will restore case-sensitive matching.  If REG_ICASE
              was specified to regcomp(), this option can be use to turn that
              off.

           n  Turn on or off special handling of the newline character.  If
              REG_NEWLINE was specified to regcomp(), this option can be use
              to turn that off.

           U  Turning on this option will make ungreedy repetitions the
              default, while turning off will make greedy repetitions the
              default.  If REG_UNGREEDY was specified to regcomp(), this
              option can be use to turn that off.

     The scope of the option change begins immediately following the right
     parenthesis, but up to the end of the enclosing subexpression (if any).
     Thus, for example, given the RE `(fu(?i)bar)baz', the `fu' portion
     matches case sensitively, `bar' matches case insensitively, and `baz'
     matches case sensitively again (since is it outside the scope of the
     subexpression in which the inline option was specified).

     The inline options syntax can be combined with the non-capturing paren-
     thesized subexpression to limit the option scope to just that of the
     subexpression.  Then, for example, `fu(?i:bar)baz' is similar to the pre-
     vious example, except for the parenthesize subexpression around
     `fu(?i)bar' in the previous example.

   Inline Comments (available for enhanced extended REs only)
     The syntax `(?#comment)' can be used to embed comments within a RE.  Note
     that comment can not contain a right parenthesis.  Also note that while
     syntactically, option characters can be added before the `#' character,
     they will be ignored.


SEE ALSO

     regex(3)

     Regular Expression Notation, IEEE Std, 1003.2, section 2.8.


BUGS

     Having two kinds of REs is a botch.

     The current IEEE Std 1003.2 (``POSIX.2'') spec says that `)' is an ordi-
     nary character in the absence of an unmatched `('; this was an uninten-
     tional 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 bracket syntax for word boundaries is incredibly ugly.

BSD                              Sept 29, 2011                             BSD

Mac OS X 10.8 - Generated Mon Sep 3 13:10:23 CDT 2012