From: Robert Klemme <shortcutter@...>
Date: 2010-01-13T23:02:23+09:00
Subject: Re: Ruby Regex

2010/1/13 Brian Candler <b.candler@pobox.com>:
> Sriram Varahan wrote:
>> Hello,
>>
>> I have a string as a = "&0&1"
>>
>> I need to pass this value with the ampersand escaped to another command
>> in my program.
>>
>> So I tried something like this:
>>
>> irb(main):038:0> a.gsub(/&/,"\\&")
>> => "&0&1"
>
> That's because \& has a special meaning in a replacement string ("the
> matched string"). Either use a block to provide the replacement value
> (which doesn't do the backslash replacement), or put two backslashes in
> the replacement string.
>
> irb(main):002:0> a.gsub(/&/) { "\\&" }
> => "\\&0\\&1"
> irb(main):003:0> a.gsub(/&/, "\\\\&")
> => "\\&0\\&1"

But it's only special when preceded by a backslash, which is special
in replacement strings.

irb(main):002:0> "123".gsub(/\d/, '<&>')
=> "<&><&><&>"
irb(main):003:0> "123".gsub(/\d/, '<\\&>')
=> "<1><2><3>"
irb(main):004:0> "123".gsub(/\d/, '<\\\\&>')
=> "<\\&><\\&><\\&>"

Another example - backslash with group index:

irb(main):011:0> "abc".gsub(/\w(.)\w/, '<1>')
=> "<1>"
irb(main):012:0> "abc".gsub(/\w(.)\w/, '<\\1>')
=> "<b>"
irb(main):013:0> "abc".gsub(/\w(.)\w/, '<\\\\1>')
=> "<\\1>"

So what you basically do here is you escape the escape so it looses
its special meaning in the replacement string. :-)

Kind regards

robert


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