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Allow complemented character class escapes within regex brackets.

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The complement-class escapes \D, \S, \W are now allowed within
bracket expressions.  There is no semantic difficulty with doing
that, but the rather hokey macro-expansion-based implementation
previously used here couldn't cope.

Also, invent "word" as an allowed character class name, thus "\w"
is now equivalent to "[[:word:]]" outside brackets, or "[:word:]"
within brackets.  POSIX allows such implementation-specific
extensions, and the same name is used in e.g. bash.

One surprising compatibility issue this raises is that constructs
such as "[\w-_]" are now disallowed, as our documentation has always
said they should be: character classes can't be endpoints of a range.
Previously, because \w was just a macro for "[:alnum:]_", such a
construct was read as "[[:alnum:]_-_]", so it was accepted so long as
the character after "-" was numerically greater than or equal to "_".

Some implementation cleanup along the way:

* Remove the lexnest() hack, and in consequence clean up wordchrs()
to not interact with the lexer.

* Fix colorcomplement() to not be O(N^2) in the number of colors
involved.

* Get rid of useless-as-far-as-I-can-see calls of element()
on single-character character element names in brackpart().
element() always maps these to the character itself, and things
would be quite broken if it didn't --- should "[a]" match something
different than "a" does?  Besides, the shortcut path in brackpart()
wasn't doing this anyway, making it even more inconsistent.

Discussion: https://postgr.es/m/2845172.1613674385@sss.pgh.pa.us
Discussion: https://postgr.es/m/3220564.1613859619@sss.pgh.pa.us
This commit is contained in:
Tom Lane
2021-02-25 13:00:40 -05:00
parent 6b40d9bdbd
commit 2a0af7fe46
10 changed files with 677 additions and 276 deletions
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25
doc/src/sgml/func.sgml
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@ -6097,6 +6097,9 @@ SELECT foo FROM regexp_split_to_table('the quick brown fox', '\s*') AS foo;
non-ASCII characters to belong to any of these classes.)
In addition to these standard character
classes, <productname>PostgreSQL</productname> defines
the <literal>word</literal> character class, which is the same as
<literal>alnum</literal> plus the underscore (<literal>_</literal>)
character, and
the <literal>ascii</literal> character class, which contains exactly
the 7-bit ASCII set.
</para>
@ -6108,9 +6111,9 @@ SELECT foo FROM regexp_split_to_table('the quick brown fox', '\s*') AS foo;
matching empty strings at the beginning
and end of a word respectively. A word is defined as a sequence
of word characters that is neither preceded nor followed by word
characters. A word character is an <literal>alnum</literal> character (as
defined by the <acronym>POSIX</acronym> character class described above)
or an underscore. This is an extension, compatible with but not
characters. A word character is any character belonging to the
<literal>word</literal> character class, that is, any letter, digit,
or underscore. This is an extension, compatible with but not
specified by <acronym>POSIX</acronym> 1003.2, and should be used with
caution in software intended to be portable to other systems.
The constraint escapes described below are usually preferable; they
@ -6330,8 +6333,7 @@ SELECT foo FROM regexp_split_to_table('the quick brown fox', '\s*') AS foo;
<row>
<entry> <literal>\w</literal> </entry>
<entry> <literal>[[:alnum:]_]</literal>
(note underscore is included) </entry>
<entry> <literal>[[:word:]]</literal> </entry>
</row>
<row>
@ -6346,21 +6348,18 @@ SELECT foo FROM regexp_split_to_table('the quick brown fox', '\s*') AS foo;
<row>
<entry> <literal>\W</literal> </entry>
<entry> <literal>[^[:alnum:]_]</literal>
(note underscore is included) </entry>
<entry> <literal>[^[:word:]]</literal> </entry>
</row>
</tbody>
</tgroup>
</table>
<para>
Within bracket expressions, <literal>\d</literal>, <literal>\s</literal>,
and <literal>\w</literal> lose their outer brackets,
and <literal>\D</literal>, <literal>\S</literal>, and <literal>\W</literal> are illegal.
(So, for example, <literal>[a-c\d]</literal> is equivalent to
The class-shorthand escapes also work within bracket expressions,
although the definitions shown above are not quite syntactically
valid in that context.
For example, <literal>[a-c\d]</literal> is equivalent to
<literal>[a-c[:digit:]]</literal>.
Also, <literal>[a-c\D]</literal>, which is equivalent to
<literal>[a-c^[:digit:]]</literal>, is illegal.)
</para>
<table id="posix-constraint-escapes-table">