using negative conditions within regular expressions

Question

Is it possible to use negative matches within gsub expressions? I want to replace strings starting by hello except those starting by hello Pe

Answer 1

As Michael told you you need a negative lookahead.

For your example is something like:

my_string.gsub(/^hello(?! peter)( .*|$)/i, '')

This will replace in cases like:

"hello"
"hello Mom"
"hello "
"hello Mom and Dad"

And will ignore things like:

"hello Peter"
"hello peter"
"hellomom"
"hello peter and tom"

Answer 2

Sounds like you want a negative lookahead:

>> "hello foo".gsub(/hello (?!peter)/, 'lala ') #=> "lala foo"
>> "hello peter".gsub(/hello (?!peter)/, 'lala ') #=> "hello peter"