Re: dollar-quoting in psql and in general

While trying to understand dollar-quoting, I found the following in psql:

select $a$ hello $a$;

behaves as expected, but psql does not like

select $a$ \ $a$;
or
select $a$ \\ $a$;

Should it? How should a dollar-quote handle:

$a$ \$a\$a $a$

?

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Philip Warner <pjw(at)rhyme(dot)com(dot)au> writes:
> While trying to understand dollar-quoting, I found the following in psql:
> select $a$ hello $a$;

> behaves as expected, but psql does not like

> select $a$ \ $a$;
> or
> select $a$ \\ $a$;

Define "does not like". The behavior seems correct to me.

> Should it? How should a dollar-quote handle:
> $a$ \$a\$a $a$

The value of that is " \$a\$a ". Backslashes are not special inside
dollar quotes --- nor is anything else, except the matching close tag.

regards, tom lane

At 12:47 PM 12/08/2004, Tom Lane wrote:
>Backslashes are not special inside
>dollar quotes --- nor is anything else, except the matching close tag

If they are not special, then shouldnt:

select $a$\$a$;

result in

?column?
\

rather than an error?

----------------------------------------------------------------
Philip Warner | __---_____
Albatross Consulting Pty. Ltd. |----/ - \
(A.B.N. 75 008 659 498) | /(@) ______---_
Tel: (+61) 0500 83 82 81 | _________ \
Fax: (+61) 03 5330 3172 | ___________ |
Http://www.rhyme.com.au | / \|
| --________--
PGP key available upon request, | /
and from pgp.mit.edu:11371 |/

Philip Warner wrote:

> While trying to understand dollar-quoting, I found the following in psql:
>
> select $a$ hello $a$;
>
> behaves as expected, but psql does not like
>
> select $a$ \ $a$;
> or
> select $a$ \\ $a$;
>
> Should it? How should a dollar-quote handle:
>
> $a$ \$a\$a $a$
>
> ?
>
>
andrew=# select $a$ \ $a$;
?column?
----------
\
(1 row)

andrew=# select $a$ \\ $a$;
?column?
----------
\\
(1 row)

It is behaving as *I* expect, and as designed. \ is not magical inside
dollar quotes, and if it were it would defeat the whole purpose of
having them.

Dollar quotes are really designed for strings that are reparsed - most
typically function bodies. The strings inside them are therefore totally
opaque, and there is no escape character. Otherwise, if we wanted the
plpgsql parsers, say, to see \ we'd have to write \\, and the idea was
to get away from \\, \', '' and friends.

cheers

andrew

At 01:07 PM 12/08/2004, Andrew Dunstan wrote:
>\ is not magical inside dollar quotes,

Sorry, I was confused by the manual: the paragraph that starts "C-style
backslash escapes are also available..." is right after the paragraphs on
dollar-quoting.

The section on dollar-quoting is also not explicit about valid tags, "zero
or more characters" is all I can see. Can you give me a definitive answer
as to what is valid? AFAICT, must be [A-Z,a-z,0-9,_]*, with non-numeric
start. Is that right?

----------------------------------------------------------------
Philip Warner | __---_____
Albatross Consulting Pty. Ltd. |----/ - \
(A.B.N. 75 008 659 498) | /(@) ______---_
Tel: (+61) 0500 83 82 81 | _________ \
Fax: (+61) 03 5330 3172 | ___________ |
Http://www.rhyme.com.au | / \|
| --________--
PGP key available upon request, | /
and from pgp.mit.edu:11371 |/

Philip Warner <pjw(at)rhyme(dot)com(dot)au> writes:
> At 12:47 PM 12/08/2004, Tom Lane wrote:
>> Backslashes are not special inside
>> dollar quotes --- nor is anything else, except the matching close tag

> If they are not special, then shouldnt:
> select $a$\$a$;
> result in
> ?column?
> \
> rather than an error?

Worksforme (TM).

regression=# select $a$\$a$;
?column?
----------
\
(1 row)

regression=#

Are you sure your psql and backend are both current?

regards, tom lane

Philip Warner <pjw(at)rhyme(dot)com(dot)au> writes:
> At 01:07 PM 12/08/2004, Andrew Dunstan wrote:
>> \ is not magical inside dollar quotes,

> Sorry, I was confused by the manual: the paragraph that starts "C-style
> backslash escapes are also available..." is right after the paragraphs on
> dollar-quoting.

The documentation for dollar-quoting is pretty awful at the moment, as
it seems to have been pasted in with no thought to context, and is
incomplete anyway. I have on my to-do list to go over it, but if
someone else beats me to it I won't pout.

> The section on dollar-quoting is also not explicit about valid tags, "zero
> or more characters" is all I can see.

Use the source, Luke ... scan.l explains

/* $foo$ style quotes ("dollar quoting")
* The quoted string starts with $foo$ where "foo" is an optional string
* in the form of an identifier, except that it may not contain "$",
* and extends to the first occurrence of an identical string.
* There is *no* processing of the quoted text.
*/
dolq_start [A-Za-z\200-\377_]
dolq_cont [A-Za-z\200-\377_0-9]
dolqdelim \$({dolq_start}{dolq_cont}*)?\$

regards, tom lane

Philip Warner wrote:

> At 01:07 PM 12/08/2004, Andrew Dunstan wrote:
>
>> \ is not magical inside dollar quotes,
>
>
> Sorry, I was confused by the manual: the paragraph that starts
> "C-style backslash escapes are also available..." is right after the
> paragraphs on dollar-quoting.
>
> The section on dollar-quoting is also not explicit about valid tags,
> "zero or more characters" is all I can see. Can you give me a
> definitive answer as to what is valid? AFAICT, must be
> [A-Z,a-z,0-9,_]*, with non-numeric start. Is that right?
>
>

Don't forget the high-bit chars too. pgsqlscan.l says:

dolq_start [A-Za-z\200-\377_]
dolq_cont [A-Za-z\200-\377_0-9]
dolqdelim \$({dolq_start}{dolq_cont}*)?\$

The scanner files for plpgsql and the backend have equivalent specs, so
this is definitive.

cheers

andrew