Re: Problem with nested StringReplace[]
- To: mathgroup at smc.vnet.net
- Subject: [mg101537] Re: Problem with nested StringReplace[]
- From: pfalloon <pfalloon at gmail.com>
- Date: Thu, 9 Jul 2009 01:59:52 -0400 (EDT)
- References: <h2i4pn$939$1@smc.vnet.net>
On Jul 2, 9:13 pm, dnquark <dnqu... at gmail.com> wrote:
> I am trying to match a substring using a regular expression, and right
> away apply some replacement rules to that substring. Suppose I want
> to match a substring "bar" and right away replace it using the rule
> {"bar" -> "baz"}. The following doesn't work: expected: "baz",
> actual: "bar"
> StringReplace[
> "foobar",
> RegularExpression["foo(bar)"] ->
> StringReplace["$1", {"bar" -> "baz"}]]
>
> Replacing the first rule with a delayed rule (:>) produces the
> expected result, but I cannot use the delayed rule -- it produces side
> effects in my code. Is there a reason that the regular rule ->
> doesn't work in this case?..
The problem with using Rule (->) is that the right hand side is
evaluated immediately. In this case,
In[609]:= StringReplace["$1", {"bar" -> "baz"}]
Out[609]= "$1"
So all you're left with is "$1", which will return "bar".
This is actually quite a typical example of where one would normally
use RuleDelayed (:>), so maybe it would be worth looking at the "side
effects" which it is producing and see whether these can be avoided?
Cheers,
Peter.