Re: Regular expression question with Postgres

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(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Mike Christensen <mike(at)kitchenpc(dot)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
>