Strange Ruby behavior when replacing with \'X string

Question

I am generating some RTF strings and need the \'bd code. I am having problem with the sub and gsub commands.

puts 'abc'.sub('a',"\\'B")  => "bcBbc"

The statement replaces the target with 'B' without the \', and copies the remainder of the string to the front. I've tried a lot of variations and it seems that the problem is the \' itself.

I have ways of getting around this, but I'd like to know whether I'm doing something fundamentally wrong or whether this is a quirk with Ruby.

Thanks

https://stackoverflow.com/questions/716767/how-to-add-a-single-backslash-character-to-a-string-in-ruby/717057 may also help. — Pavel Oganesyan, Apr 20 '20 at 16:34

score 2 · Accepted Answer · edited Apr 20 '20 at 17:26

From the Ruby documentation:

Similarly, \&, \', \`, and \+ correspond to special variables, $&, $', $`, and $+, respectively.

And here, the documentation goes on to say:

$~ is equivalent to ::last_match;

$& contains the complete matched text;

$` contains string before match;

$' contains string after match;

$1, $2 and so on contain text matching first, second, etc capture group;

$+ contains last capture group.

Example:
m = /s(\w{2}).*(c)/.match('haystack') #=> #<MatchData "stac" 1:"ta" 2:"c">
$~                                    #=> #<MatchData "stac" 1:"ta" 2:"c">
Regexp.last_match                     #=> #<MatchData "stac" 1:"ta" 2:"c">

$&      #=> "stac"
        # same as m[0]
$`      #=> "hay"
        # same as m.pre_match
$'      #=> "k"
        # same as m.post_match
$1      #=> "ta"
        # same as m[1]
$2      #=> "c"
        # same as m[2]
$3      #=> nil
        # no third group in pattern
$+      #=> "c"
        # same as m[-1]

So, \' in a substitution replacement string has a special meaning. It means "The portion of the original string after the match" - which, in this case, is "bc".

So rather than getting \'Bbc, you get bcBbc

Therefore unfortunately, in this odd scenario, you'd need to double-escape the backslashes:

puts 'abc'.sub('a',"\\\\'B")  => "\'Bbc"

Thanks for the reformatting @3limin4t0r, I couldn't work out how to properly format code containing a backtick! — Tom Lord, Apr 21 '20 at 09:57

Strange Ruby behavior when replacing with \'X string

1 Answers1