From: "Michael W. Ryder" <_mwryder@...>
Date: 2007-07-20T17:34:58+09:00
Subject: Re: Is there a replacement for sub?

Robert Klemme wrote:
> 2007/7/20, Michael W. Ryder <_mwryder@worldnet.att.net>:
>> Morton Goldberg wrote:
>> > On Jul 19, 2007, at 11:00 PM, Michael W. Ryder wrote:
>> >
>> >> I was trying to come up with a way to remove x instances of a
>> >> character from a string and came up with a problem.  If I enter:
>> >>
>> >> a = "a b c d e f"
>> >> for i in 1..3
>> >>   a = a.sub!(' ', '')
>> >> end
>> >> puts a  ==> returns 'abcd e f' which is correct.
>> >>
>> >> But if I enter:
>> >>
>> >> a = "a b c d e f"
>> >> for i in 1..10
>> >>   a = a.sub!(' ', '')
>> >> end
>> >> puts a  ==> returns error.rb:3: private method `sub!' called for
>> >> nil:NilClass (NoMethodError, and a is now nil.
>> >>
>> >> What I am looking for is a way to remove the first n instances of a
>> >> blank from the string without wiping out the string if it does not
>> >> contain at least n blanks.  I assume there is a way to do this with
>> >> regular expressions, but have not found it yet.  This is something an
>> >> editor I liked, UCEDIT, on the CDC Cyber had in the 70's.
>> >
>> > How about this?
>> >
>> > n = 3
>> > "a b c d e f".sub(/(\S\s){#{n}}/) { |m| m.delete(" ") } # => "abcd e f"
>> > n = 10
>> > "a b c d e f".sub(/(\S\s){#{n}}/) { |m| m.delete(" ") } # => "a b c 
>> d e f"
>> >
>> > Regards, Morton
>> >
>>
>> Is there nothing in regular expressions where you can tell it to do
>> something up to n times?
> 
> There is - kind of.  You can use {} to give repetition counts.  You can 
> do this
> 
> irb(main):004:0> a = "a b c d e f"
> => "a b c d e f"
> irb(main):005:0> a.sub(/(?: [^ ]*){3}/) {|m| m.gsub(/ /, '') }
> => "abcd e f"
> irb(main):006:0>
> 
> Kind regards
> 
> robert
> 
Unfortunately this is much more complicated and much harder to 
understand, and debug.  The editor I mentioned had an argument you 
passed to the expression that told it to do it one time if it was 
absent, n number of times, or till the end.  Since this was 30 years ago 
I expected that something like this hadn't been dropped in the interim.