Question

I was trying to write a regexp to replace all occurrences of with unless the is already preceeded immediately by a . I m doing this in Ruby 1.8.6 which doesn t support look behind in regexps so I tried:

# try to replace 
 proceeded by anything other than 
 with 

str.gsub(/([^
])
/, "\1
") # \1 is the captured character to be kept

Given a string One Two Three the intention was for to be replaced with and the existing between Two and Three to be left unmodified. However only the first of the two match. i.e. the result is:

 "One

Two
Three"

I tried this in a couple of other regexp engines with the same result.

In the end I was able to solve this by using Oniguruma (which does support positive and negative look behind) instead of Ruby s built in regexps but I am still interested in why my alternative approach didn t work as I expected.

Thanks for any answers.

Answer 1

Just writing to explain (rsp s comment says the same thing) why your original regex didn t work. The regex engine first matches ([^ ]) at the ^ characters:

One

Two
Three
   ^^^^

After the first replacement, the regex engine is at the ^:

One

Two
Three
       ^

It s now trying to match ([^ ]), but there is no character that is not at the caret position. So it won t match until the between Two and Three.

Answer 2

You could simply replace ? with :

s = s.gsub(/
?
/, "
")

That way, all s and s are replaced by .

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