Newbie: Strings and RegExp

Hello,
given an instance of String, str, how do you select/copy the section of
the string between two given strings START_STRING and END_STRING?

I used something like:
re = /#{STRING_BEFORE}(regular_expression)#{STRING_AFTER}/
str =~ re
middle = $1
where regular_expression is supposed to be a regular expression
selecting any character (digits, space, newline, character,
anything…).

Is this the right method? And what should I use for regular_expression?
I tried (.|\n)* but I get a “BUS error”. Any other way?

Or maybe is there a method which does what I need?

(Note: I am using Ruby 1.6.4 on Solaris)

Thank you

Diego

On 19.10.2006 17:56, Diego V. wrote:

anything…).

Is this the right method? And what should I use for regular_expression?
I tried (.|\n)* but I get a “BUS error”. Any other way?

First, you do not need this alternation if you use /m (multiline mode).
This regexp probably does an enourmous amount of backtracking because
of this alternation. What’s in STRING_BEFORE and STRING_AFTER? If they
contain regexp metacharacters you should probably do

/#{Regexp.escape STRING_BEFORE}…

Depending on the length of your string, you might be better off with
something like this:

array = str.split(/(#{Regexp.escape STRING_BEFORE}|#{Regexp.escape
STRING_AFTER})/)

And then extract the middle portion from the resulting array.

Or maybe is there a method which does what I need?

(Note: I am using Ruby 1.6.4 on Solaris)

That version is ancient. It might even have some bugs which are fixed
in never versions. I strongly recommend to upgrade if possible.

Kind regards

robert

Diego V. wrote:

given an instance of String, str, how do you select/copy the section of
the string between two given strings START_STRING and END_STRING?

Here’s one way:
STRING_BEFORE = “hello”
STRING_AFTER = “goodbye”
re = /#{STRING_BEFORE}(.+?)#{STRING_AFTER}/mo
str = “I just wanted to say hello to you before I say goodbye for the
final time.”
str << " Oh, hello and goodbye again."
p str.scan( re ).flatten
#=> [" to you before I say ", " and "]

You should also be able to do this with split. Then you can use
strings as well as regexes:

class String
  def match_between(head, tail)
    strings = []
    s = split(head, 2)[1]
    while (s)
      match, rest = s.split(tail,2)
      return strings unless rest
      strings << match
      s = rest.split(head, 2)[1]
    end
    strings
  end
end

So with the same string as before,

str = "I just wanted to say hello to you before I say goodbye for

the final time."
str << " Oh, hello and goodbye again."

you get:

p str.match_between(/hello/,/goodbye/)
# => [" to you before I say ", " and "]

and also

p str.match_between('a','e')
# => ["nt", "y h", "y goodby", "l tim", "nd goodby"]

I’m sure you could also tweek it to operate in a manner which doesn’t
consume the rest of the string, i.e.: the ‘g’ flag.

That version is ancient. It might even have some bugs which are fixed
in never versions. I strongly recommend to upgrade if possible.

Yes, I’d like to but it’s the Univeristy computer so I have to live
with what there is. Sigh…

Diego

Here’s one way:
STRING_BEFORE = “hello”
STRING_AFTER = “goodbye”
re = /#{STRING_BEFORE}(.+?)#{STRING_AFTER}/mo
str = “I just wanted to say hello to you before I say goodbye for the
final time.”
str << " Oh, hello and goodbye again."
p str.scan( re ).flatten
#=> [" to you before I say ", " and "]

Perfect, it works. Thanks a lot, and to Robert and Louis too, thanks.

So basically I gather the main problem was using the /mo. Right? That
alone, with the simpler regular expression seems to do the job. (maybe
before it was running out of memory? is that what bus error means?)

However here you are using /mo instead of /m. What does the extra o
mean? And where can I read more about this topic? I’ve checked on the
Pragmatic Programmer book (the one online) but I can’t find any info on
that.

Thanks again.

Diego

Diego V. wrote:

That version is ancient. It might even have some bugs which are fixed
in never versions. I strongly recommend to upgrade if possible.

Yes, I’d like to but it’s the Univeristy computer so I have to live
with what there is. Sigh…

:slight_smile: I believe you can compile Ruby in a way and install it completely
in your home directory. That of course works only if your quota is high
enough. Maybe worth a try.

Good luck!

robert

You absolutely can compile and install to home, just set the --prefix to
something like
–prefix=/usr/local/home/yours/ruby-1_8/

Then make sure the bin directory is in your path before the system
installed ruby that way env ruby picks up your new install.

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