Substituting Variables

ruby-talk
1

Hello,

I am running into difficulties trying to use a variable in a substitution. Here's an example:

var = 'October'
'30 May 2007'.sub( /(\d+)\s(\w+)\s(\d+)/, '\1 #{var} \3' )

Here I would like to get '30 October 2007'. The back-references, i.e. \1 and \3, only work within single quotation marks. But the interpolation only works within double quotation marks. What to do?

Here's an example in IRB:

>> var = 'October'
>> re = /(\d+)\s(\w+)\s(\d+)/

>> '30 May 2007'.sub(re, '\1 #{var} \3')
=> "30 \#{var} 2007"

>> '30 May 2007'.sub(re, "\1 #{var} \3")
=> "\001 October \003"

>> '30 May 2007'.sub(re, "#{$1} #{var} #{$3}")
=> "30 October 2007"

Actually that last one does what I want -- but according to the Pickaxe[1] $1 and friends aren't supposed to work and in fact it doesn't work in my Ruby class. So I think IRB is flattering to deceive.

[1] String.gsub: "If a string is used as the replacement, special variables from the match (such as $& and $1) cannot be substituted into it, as substitution into the string occurs before the pattern match starts. However, the sequences \1, \2, and so on may be used to interpolated successive groups in the match."

I have found a workaround but I'm hoping there's a better way:

>> '30 May 2007'.sub(re, '\1' + " #{var} " + '\3')
=> "30 October 2007"

Any insight would be much appreciated.

Thanks and regards,
Andy Stewart

2

Hi --

···

On Thu, 31 May 2007, Andrew Stewart wrote:

Hello,

I am running into difficulties trying to use a variable in a substitution. Here's an example:

var = 'October'
'30 May 2007'.sub( /(\d+)\s(\w+)\s(\d+)/, '\1 #{var} \3' )

Here I would like to get '30 October 2007'. The back-references, i.e. \1 and \3, only work within single quotation marks. But the interpolation only works within double quotation marks. What to do?

Try:

   "\\1 #{var} \\3"

David

--
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A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black\)
    (See what readers are saying! http://www.rubypal.com/r4rrevs.pdf\)
Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
A. Ruby Power and Light, LLC (http://www.rubypal.com)

3

Use double slashes.

irb(main):001:0> var = 'October'
irb(main):002:0> '30 May 2007'.sub( /(\d+)\s(\w+)\s(\d+)/, "\\1 #{var} \\3" )
=> "30 October 2007"

···

On 5/30/07, Andrew Stewart <boss@airbladesoftware.com> wrote:

I am running into difficulties trying to use a variable in a
substitution. Here's an example:

var = 'October'
'30 May 2007'.sub( /(\d+)\s(\w+)\s(\d+)/, '\1 #{var} \3' )

Here I would like to get '30 October 2007'. The back-references,
i.e. \1 and \3, only work within single quotation marks. But the
interpolation only works within double quotation marks. What to do?

--
Luis Parravicini
http://ktulu.com.ar/blog/

4

Hello Andrew,

Use double quotes and escape slashes.
'30 May 2007'.sub( /(\d+)\s(\w+)\s(\d+)/, "\\1 #{var} \\3" )

Wednesday, May 30, 2007, 8:09:29 PM, you wrote:

Hello,

I am running into difficulties trying to use a variable in a
substitution. Here's an example:

var = 'October'
'30 May 2007'.sub( /(\d+)\s(\w+)\s(\d+)/, '\1 #{var} \3' )

Here I would like to get '30 October 2007'. The back-references,
i.e. \1 and \3, only work within single quotation marks. But the
interpolation only works within double quotation marks. What to do?

Here's an example in IRB:

>>> var = 'October'
>>> re = /(\d+)\s(\w+)\s(\d+)/

>>> '30 May 2007'.sub(re, '\1 #{var} \3')
=>> "30 \#{var} 2007"

>>> '30 May 2007'.sub(re, "\1 #{var} \3")
=>> "\001 October \003"

>>> '30 May 2007'.sub(re, "#{$1} #{var} #{$3}")
=>> "30 October 2007"

Actually that last one does what I want -- but according to the
Pickaxe[1] $1 and friends aren't supposed to work and in fact it
doesn't work in my Ruby class. So I think IRB is flattering to deceive.

[1] String.gsub: "If a string is used as the replacement, special
variables from the match (such as $& and $1) cannot be substituted
into it, as substitution into the string occurs before the pattern
match starts. However, the sequences \1, \2, and so on may be used
to interpolated successive groups in the match."

I have found a workaround but I'm hoping there's a better way:

>>> '30 May 2007'.sub(re, '\1' + " #{var} " + '\3')
=>> "30 October 2007"

···

Any insight would be much appreciated.

Thanks and regards,
Andy Stewart

--
Best regards,
kane mailto:kane.sub@inbox.uz

5

Same workaround, but easier to read (to me, anyway):
'30 May 2007'.sub(re, '\1 ' << var << ' \3')
Since you're assembling a string, there's no need to interpolate.

And, if you're not using the middle capture group, do away with it. I
also captured the spaces in the flanking groups.

re2 = /(\d+\s)\w+(\s\d+)/
'30 May 2007'.sub(re2, '\1' << var << '\2')

In 1.9, Oniguruma will give us zero-width lookahead and lookbehind.

-A

···

On 5/30/07, Andrew Stewart <boss@airbladesoftware.com> wrote:

Hello,

I am running into difficulties trying to use a variable in a
substitution. Here's an example:

var = 'October'
'30 May 2007'.sub( /(\d+)\s(\w+)\s(\d+)/, '\1 #{var} \3' )

Here I would like to get '30 October 2007'. The back-references,
i.e. \1 and \3, only work within single quotation marks. But the
interpolation only works within double quotation marks. What to do?

Here's an example in IRB:

>> var = 'October'
>> re = /(\d+)\s(\w+)\s(\d+)/

>> '30 May 2007'.sub(re, '\1 #{var} \3')
=> "30 \#{var} 2007"

>> '30 May 2007'.sub(re, "\1 #{var} \3")
=> "\001 October \003"

>> '30 May 2007'.sub(re, "#{$1} #{var} #{$3}")
=> "30 October 2007"

Actually that last one does what I want -- but according to the
Pickaxe[1] $1 and friends aren't supposed to work and in fact it
doesn't work in my Ruby class. So I think IRB is flattering to deceive.

[1] String.gsub: "If a string is used as the replacement, special
variables from the match (such as $& and $1) cannot be substituted
into it, as substitution into the string occurs before the pattern
match starts. However, the sequences \1, \2, and so on may be used
to interpolated successive groups in the match."

I have found a workaround but I'm hoping there's a better way:

>> '30 May 2007'.sub(re, '\1' + " #{var} " + '\3')
=> "30 October 2007"

Any insight would be much appreciated.

Thanks and regards,
Andy Stewart

6

Hi David, Luis and Kane,

Try:

  "\\1 #{var} \\3"

That works swimmingly. Such fast replies too. Thank you very much!

Regards,
Andy Stewart

7

Hello,

I am running into difficulties trying to use a variable in a
substitution. Here's an example:

var = 'October'
'30 May 2007'.sub( /(\d+)\s(\w+)\s(\d+)/, '\1 #{var} \3' )

Here I would like to get '30 October 2007'. The back-references,
i.e. \1 and \3, only work within single quotation marks. But the
interpolation only works within double quotation marks. What to do?

Here's an example in IRB:

>> var = 'October'
>> re = /(\d+)\s(\w+)\s(\d+)/

>> '30 May 2007'.sub(re, '\1 #{var} \3')
=> "30 \#{var} 2007"

>> '30 May 2007'.sub(re, "\1 #{var} \3")
=> "\001 October \003"

>> '30 May 2007'.sub(re, "#{$1} #{var} #{$3}")
=> "30 October 2007"

Actually that last one does what I want -- but according to the
Pickaxe[1] $1 and friends aren't supposed to work and in fact it
doesn't work in my Ruby class. So I think IRB is flattering to deceive.

[1] String.gsub: "If a string is used as the replacement, special
variables from the match (such as $& and $1) cannot be substituted
into it, as substitution into the string occurs before the pattern
match starts. However, the sequences \1, \2, and so on may be used
to interpolated successive groups in the match."

I have found a workaround but I'm hoping there's a better way:

>> '30 May 2007'.sub(re, '\1' + " #{var} " + '\3')
=> "30 October 2007"

Any insight would be much appreciated.

Thanks and regards,
Andy Stewart

Same workaround, but easier to read (to me, anyway):
'30 May 2007'.sub(re, '\1 ' << var << ' \3')
Since you're assembling a string, there's no need to interpolate.

Um, what? Assembling strings is exactly what string interpolation is used for (what else would it be?). Granted, there are multiple ways to do it but your statement seems to be a bit off the mark.

And, if you're not using the middle capture group, do away with it. I
also captured the spaces in the flanking groups.

re2 = /(\d+\s)\w+(\s\d+)/
'30 May 2007'.sub(re2, '\1' << var << '\2')

Good point.

Kind regards

  robert

···

On 30.05.2007 17:28, Alex LeDonne wrote:

On 5/30/07, Andrew Stewart <boss@airbladesoftware.com> wrote:

8

Sorry for my imprecision, and thank you. I meant to say that if you're
_concatenating_, then there's no need to interpolate. The
double-quote, double-backslash solution presented by others uses
interpolation instead of concatenation.

In hindsight, I suppose the backslash construction is also a form of
interpolation, so my comment didn't make much sense after all. Ah,
well. At least the modified regex worked. :slight_smile:

-Alex

···

On 5/30/07, Robert Klemme <shortcutter@googlemail.com> wrote:

On 30.05.2007 17:28, Alex LeDonne wrote:
> Same workaround, but easier to read (to me, anyway):
> '30 May 2007'.sub(re, '\1 ' << var << ' \3')
> Since you're assembling a string, there's no need to interpolate.

Um, what? Assembling strings is exactly what string interpolation is
used for (what else would it be?). Granted, there are multiple ways to
do it but your statement seems to be a bit off the mark.