Inconsistent regexp behavior

Ruby
1

I have a script with two lines:

puts “toads suck”.gsub(/(\s)/, “\#{$1}”)

puts " a ".gsub(/(\s), “\#{$1}”)

I get broken output for the first:

toads\suck

\ a\

Can anyone explain why this is happening. I’m convinced it shouldn’t
ever
happen.

–
“Hey brother Christian with your high and mighty errand, Your actions
speak
so loud, I can’t hear a word you’re saying.”

-Greg Graffin (Bad Religion)

2

On 14.08.2008 18:40, Glen H. wrote:

toads\suck

\ a\

Can anyone explain why this is happening. I’m convinced it shouldn’t ever
happen.

I’m sorry, but you’re wrong. :slight_smile: When you pass “\#{$1}” to gsub the
string is evaluated once and even worse, the value of $1 at time of
call has nothing to do with this gsub operation. gsub did not even
start matching at this point in time.

The second replacement apparently works because it gets $1 from the
first execution…

You also do not need the grouping because the group covers the whole
match anyway. You rather want a special replacement expression:

irb(main):001:0> “toads suck”.gsub(/\s/, ‘\\\&’)
=> “toads\ suck”
irb(main):002:0> puts “toads suck”.gsub(/\s/, ‘\\\&’)
toads\ suck
=> nil

Note, the high number of backslashes is necessary because there are two
levels of escaping: first the string, i.e. you need two backslashes to
have one backslash in the string. Then you need to escape the backslash
you want to insert into the replacement because backslash is a
metacharacter in the substitution string itself.

Kind regards

robert

3

Glen H. wrote:

puts “toads suck”.gsub(/(\s)/, “\#{$1}”)

Think about in what order the elements of this expression get evaluated.
Then
think about what the value of $1 is when “\#{$1}” gets evaluated.

HTH,
Sebastian

4

On Thu, Aug 14, 2008 at 10:40 AM, Glen H. [email protected]
wrote:

Oops. Typo, I should probably copy and paste next time. The second
gsub is
actually:

puts " a ".gsub(/(\s)/, “\#{$1}”)

–
“Hey brother Christian with your high and mighty errand, Your actions
speak
so loud, I can’t hear a word you’re saying.”

-Greg Graffin (Bad Religion)

5

On Thu, Aug 14, 2008 at 11:01 AM, Glen H. [email protected]
wrote:

I get broken output for the first:
"Hey brother Christian with your high and mighty errand, Your actions
puts " a ".gsub(/(\s)/, “\#{$1}”)

–
“Hey brother Christian with your high and mighty errand, Your actions speak
so loud, I can’t hear a word you’re saying.”

-Greg Graffin (Bad Religion)

Thanks guys, now I know why.

–
“Hey brother Christian with your high and mighty errand, Your actions
speak
so loud, I can’t hear a word you’re saying.”

-Greg Graffin (Bad Religion)

6

From: Glen H. [mailto:[email protected]]

puts “toads suck”.gsub(/(\s)/, “\#{$1}”)

toads\suck

Can anyone explain why this is happening. I’m convinced it

shouldn’t ever happen.

it’s a common surprise, but ruby has a solution for that; try the block
form

irb(main):002:0> puts “toads suck”.gsub(/(\s)/) { “\#{$1}” }
toads\ suck
=> nil

no surprise.

kind regards -botp

7

2008/8/15 Peña, Botp [email protected]:

=> nil
I was thinking about including a statement about the block form but
decided against - now you made me write it anyway… :slight_smile:

IMHO the block form should be restricted to cases where substitutions
should be done that cannot be implemented with simple pattern
replacement (e.g. inserting a match counter, doing lookups somewhere
etc.). This is mainly because I find the non block form more
appropriate if you are only doing pattern replacements but it also has
the nice side effect of being more efficient. Use the right tool for
the job.

Kind regards

robert