Thread: Regular expression question with Postgres
I'm curious why this query returns 0:
SELECT 'AAA' ~ '^A{,4}$'
Yet, this query returns 1:
SELECT 'AAA' ~ '^A{0,4}$'
Is this a bug with the regular expression engine?
Mike Christensen-2 wrote
> I'm curious why this query returns 0:
>
> SELECT 'AAA' ~ '^A{,4}$'
>
> Yet, this query returns 1:
>
> SELECT 'AAA' ~ '^A{0,4}$'
>
> Is this a bug with the regular expression engine?
Apparently since "{,#}" is not a valid regexp expression the engine simply
interprets it as a literal and says 'AAA' != 'A{,4}'
http://www.postgresql.org/docs/9.3/interactive/functions-matching.html#FUNCTIONS-POSIX-REGEXP
Table 9-13. Regular Expression Quantifiers
Note the all of the { } expressions have a lower bound (whether explicit or
implied).
David J.
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Yea seems right. I was testing the expression on Rubular (Which uses the Ruby parser) and it worked. I guess Ruby allows this non-standard expression with the missing lower bounds. Every reference I could find, though, agrees only the upper bound is optional.
On Thu, Jul 24, 2014 at 1:42 PM, David G Johnston <david.g.johnston@gmail.com> wrote:
Mike Christensen-2 wroteApparently since "{,#}" is not a valid regexp expression the engine simply> I'm curious why this query returns 0:
>
> SELECT 'AAA' ~ '^A{,4}$'
>
> Yet, this query returns 1:
>
> SELECT 'AAA' ~ '^A{0,4}$'
>
> Is this a bug with the regular expression engine?
interprets it as a literal and says 'AAA' != 'A{,4}'
http://www.postgresql.org/docs/9.3/interactive/functions-matching.html#FUNCTIONS-POSIX-REGEXP
Table 9-13. Regular Expression Quantifiers
Note the all of the { } expressions have a lower bound (whether explicit or
implied).
David J.
--
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Mike Christensen <mike@kitchenpc.com> writes:
> I'm curious why this query returns 0:
> SELECT 'AAA' ~ '^A{,4}$'
> Yet, this query returns 1:
> SELECT 'AAA' ~ '^A{0,4}$'
> Is this a bug with the regular expression engine?
Our regex documentation lists the following variants of bounds syntax:
{m}
{m,}
{m,n}
Nothing about {,n}. I rather imagine that the engine is deciding that
that's just literal text and not a bounds constraint ...
regression=# SELECT 'A{,4}' ~ '^A{,4}$';
?column?
----------
t
(1 row)
... yup, apparently so.
A look at the POSIX standard says that it has the same idea of what
is a valid bounds constraint:
When an ERE matching a single character or an ERE enclosed in
parentheses is followed by an interval expression of the format
"{m}", "{m,}", or "{m,n}", together with that interval expression
it shall match what repeated consecutive occurrences of the ERE
would match. The values of m and n are decimal integers in the
range 0 <= m<= n<= {RE_DUP_MAX}, where m specifies the exact or
minimum number of occurrences and n specifies the maximum number
of occurrences. The expression "{m}" matches exactly m occurrences
of the preceding ERE, "{m,}" matches at least m occurrences, and
"{m,n}" matches any number of occurrences between m and n,
inclusive.
regards, tom lane
Yea looks like Postgres has it right, well.. per POSIX standard anyway. JavaScript also has it right, as does Python and .NET. Ruby is just weird.
On Thu, Jul 24, 2014 at 1:57 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Our regex documentation lists the following variants of bounds syntax:Mike Christensen <mike@kitchenpc.com> writes:
> I'm curious why this query returns 0:
> SELECT 'AAA' ~ '^A{,4}$'
> Yet, this query returns 1:
> SELECT 'AAA' ~ '^A{0,4}$'
> Is this a bug with the regular expression engine?
{m}
{m,}
{m,n}
Nothing about {,n}. I rather imagine that the engine is deciding that
that's just literal text and not a bounds constraint ...
regression=# SELECT 'A{,4}' ~ '^A{,4}$';
?column?
----------
t
(1 row)
... yup, apparently so.
A look at the POSIX standard says that it has the same idea of what
is a valid bounds constraint:
When an ERE matching a single character or an ERE enclosed in
parentheses is followed by an interval expression of the format
"{m}", "{m,}", or "{m,n}", together with that interval expression
it shall match what repeated consecutive occurrences of the ERE
would match. The values of m and n are decimal integers in the
range 0 <= m<= n<= {RE_DUP_MAX}, where m specifies the exact or
minimum number of occurrences and n specifies the maximum number
of occurrences. The expression "{m}" matches exactly m occurrences
of the preceding ERE, "{m,}" matches at least m occurrences, and
"{m,n}" matches any number of occurrences between m and n,
inclusive.
regards, tom lane