# Backslash sequences\\1\\2 in regexs (backreferences)

**URL:** <https://www.ruby-forum.com/t/backslash-sequences-1-2-in-regexs-backreferences/119006>\
**Category:** Ruby\
**Created:** [November 2, 2007, 2:47am UTC](https://www.ruby-forum.com/t/backslash-sequences-1-2-in-regexs-backreferences/119006 "2007-11-02T02:47:05Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![7stud](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/7/57b2e6/32.png) [@7stud](https://www.ruby-forum.com/u/7stud)\
**Post date:** [November 2, 2007, 2:47am UTC](https://www.ruby-forum.com/t/backslash-sequences-1-2-in-regexs-backreferences/119006/1 "2007-11-02T02:47:05Z")

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Is this behavior documented anywhere:

1. 

puts “fred:smith”.gsub(/(\w+):(\w+)/, ‘\2, \1’)

–output:–  
smith, fred

1. 

puts “abc”.gsub(/a(b)©/, “a\2\1”)

–output:–  
a

The double quotes surrounding the replacement string cause the backslash  
sequences to stop working. With single quotes the backslash sequences  
work. I can’t find anything in pickaxe2 about that. .My understanding  
was that double quotes allowed for more substitutions than single  
quotes. This appears to be a case where double quotes allow fewer  
substitutions than single quotes.

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**Author:** ![7stud](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/7/57b2e6/32.png) [@7stud](https://www.ruby-forum.com/u/7stud)\
**Post date:** [November 2, 2007, 3:32am UTC](https://www.ruby-forum.com/t/backslash-sequences-1-2-in-regexs-backreferences/119006/2 "2007-11-02T03:32:00Z")

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On 1-Nov-07, at 9:47 PM, 7stud – wrote:

> ## substitutions than single quotes.
> 
> Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/).

The double quotes interpolate the \1 and \2 as characters before gsub  
ever sees it.

ratdog:~ mike$ ruby -e ‘puts “abc”.gsub(/a(b)(c)/, “a\2\1”)’ | od -c  
0000000 a 002 001 \n  
0000004

ratdog:~ mike$ irb  
irb(main):001:0\> ‘a\1\2’.length  
=\> 5  
irb(main):002:0\> “a\1\2”.length  
=\> 3  
irb(main):003:0\> “a\2\1”  
=\> “a\002\001”

the \2 and \1 are interpolated into two single characters in the  
double quotes.

Table 22.2 in The Basic Types says \nnn goes to Octal nnn, and here  
you see 8 (not a valid octal digit) doesn’t get treated the same way  
as 1 and 2:

irb(main):004:0\> “a\2\1\8”  
=\> “a\002\0018”

Hope this helps,

Mike

–

Mike S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
[http://www.stok.ca/~mike/](http://www.stok.ca/~mike/)

The “`Stok’ disclaimers” apply.

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<div class="post-metadata">

**Author:** ![7stud](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/7/57b2e6/32.png) [@7stud](https://www.ruby-forum.com/u/7stud)\
**Post date:** [November 2, 2007, 5:01am UTC](https://www.ruby-forum.com/t/backslash-sequences-1-2-in-regexs-backreferences/119006/3 "2007-11-02T05:01:26Z")

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On Nov 1, 7:47 pm, removed\_email\_address@domain.invalid wrote:

> Is this behavior documented anywhere:

Yes. In many Ruby books, in at least one Ruby FAQ, and many, many  
times on the ruby mailing list/forum/newsgroup.

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<div class="post-metadata">

**Author:** ![7stud](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/7/57b2e6/32.png) [@7stud](https://www.ruby-forum.com/u/7stud)\
**Post date:** [November 2, 2007, 5:59am UTC](https://www.ruby-forum.com/t/backslash-sequences-1-2-in-regexs-backreferences/119006/4 "2007-11-02T05:59:34Z")

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Mike S. wrote:

> Table 22.2 in The Basic Types says \nnn goes to Octal nnn,

Ah. So, \1 and \2 are interpreted as octal character codes. I was  
using the following puts statement to debug:

puts “abc”.gsub(/a(b)©/, “a\2\1”) + “\<—”

–output:–  
a\<—

I should have been using:

p “abc”.gsub(/a(b)©/, “a\2\1”)

–output:–  
“a\002\001”

Since the ascii codes 1 and 2 represent non-printable characters, I got  
no output for them using puts.

My question stemmed from this passage about gsub() in pickaxe2 on p.  
613:

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

That makes it sound like \1 and \2 can be freely used in the replacement  
string. There is no mention of the fact that single quotes are required  
to keep them from being interpreted as chars written in octal. That  
description is very misleading

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<div class="post-metadata">

**Author:** ![7stud](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/7/57b2e6/32.png) [@7stud](https://www.ruby-forum.com/u/7stud)\
**Post date:** [November 2, 2007, 6:34am UTC](https://www.ruby-forum.com/t/backslash-sequences-1-2-in-regexs-backreferences/119006/5 "2007-11-02T06:34:17Z")

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On Nov 2, 2007, at 12:59 AM, 7stud – wrote:

> –output:–  
> got
> 
> That makes it sound like \1 and \2 can be freely used in the  
> replacement  
> string. There is no mention of the fact that single quotes are  
> required  
> to keep them from being interpreted as chars written in octal. That  
> description is very misleading

No, it’s not, That single quotes are required has nothing to do with  
gsub. It’s something you should know from your understanding of how  
the Ruby interpreter handles double quoted strings. As Mike S. said  
the string literal is converted to “a\002\001” long before gsub is  
called.

Regards, Morton

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<div class="post-metadata">

**Author:** ![7stud](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/7/57b2e6/32.png) [@7stud](https://www.ruby-forum.com/u/7stud)\
**Post date:** [November 2, 2007, 6:51am UTC](https://www.ruby-forum.com/t/backslash-sequences-1-2-in-regexs-backreferences/119006/6 "2007-11-02T06:51:32Z")

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Morton G. wrote:

> On Nov 2, 2007, at 12:59 AM, 7stud – wrote:
> 
> > –output:–  
> > got
> > 
> > That makes it sound like \1 and \2 can be freely used in the  
> > replacement  
> > string. There is no mention of the fact that single quotes are  
> > required  
> > to keep them from being interpreted as chars written in octal. That  
> > description is very misleading
> 
> No, it’s not, That single quotes are required has nothing to do with  
> gsub. It’s something you should know from your understanding of how  
> the Ruby interpreter handles double quoted strings. As Mike S. said  
> the string literal is converted to “a\002\001” long before gsub is  
> called.
> 
> Regards, Morton  
> You should simply use “double-quote-double-quote”

irb(main):001:0\> puts “fred:smith”.gsub(/(\w+):(\w+)/, ‘\2, \1’)  
smith, fred

Wolfgang Nádasi-Donner