Re: [Pan-users] Re: global scores

[Rhialto]

On Sat 12 May 2007 at 11:11:33 +0000, Duncan wrote:
> [] indicates a character class.  Any of the enumerated characters will 
> match.  A range may be indicated with a dash (which can be matched 
> literally by placing it first, after a ^ if any), and a ^ as the first 
> character negates.  As with a -, a ] must be placed first (or escaped) as 

This (about characters being escaped within []) is a widespread
misunderstanding, but it is Not True(tm).

aside:  Worse, some people who re-implemented regular expression
        matchers also misunderstood it, making even more of a mess,
        because it is now "sometimes true, sometimes false".  Regular
        expressions in Perl have to follow some Perl rules too, so the
        "true" regular expression sometimes needs an extra backslash
        just so that Perl understands where it starts and ends...
        creating even more confusion. I don't know how that is handled
        in the "pcre" library.

Within [] (called a character class) all characters lose their other
meaning and only some characters have special meaning:

^ at the start for negation
- for a range
] to end the character class.

that is all (apart from the abomination of POSIX character classes
which was invented much later than the rest, and which complicates
matters needlessly).

To include a - literally, use it at the start or end (because it would
not be a complete range specification).
To include a ] literally, use it at the start (because it would make the
character class empty and thus useless).

So [^]-] is a character class matching all characters except ] and -,
and [][] is a character class matching ] and [
and []^-] is a character class matching ] ^ and -.

>From the re_format(7) manual page which deals with the POSIX character
classes too (you don't really want to know about them unless there is no
other way):

       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  charac-
       ters  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  spe-
       cial  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  o
       and  ^  are  the  members  of  an  equivalence  class,  then `[[=o=]]',
       `[[=^=]]', and `[o^]' 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
       expressions `[[:<:]]' and `[[:>:]]' match the null string at the begin-
       ning  and  end of a word respectively.  A word is defined as a sequence
       of word characters which is neither preceded nor followed by word char-
       acters.   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 POSIX 1003.2, and should be used with caution in soft-
       ware intended to be portable to other systems.

-Olaf.
-- 
___ Olaf 'Rhialto' Seibert      -- You author it, and I'll reader it.
\X/ rhialto/at/xs4all.nl        -- Cetero censeo "authored" delendum esse.