Ruby gsub的正则表达式！ 锚点不匹配

Question

The following regular expression should work in Ruby, but it doesn't. Any ideas on how to fix it, so it can be used in a .gsub! statement in a loop?

textfield.gsub!( /(http:\/\/){0}www\./, 'http://www.' )

{0} should allow to match the first part zero times, but it does not:

'http://www.company1.com
 http://www.company2.com'.gsub!( /(http:\/\/){0}www\./, 'http://www.' )

=> " http://http://www.company1.com http://http://www.company2.com

in this example the regexp should not match, and leave the input string unmodified!

any ideas on how to make this work?

this looks like a bug in Ruby's regexp processing

I admit that I'm trying to generously interpret the semantics of {n} to include n = 0 :)

Answer 1

The trouble is that /(http:\\/\\/){0}/ matches the start of any string. In fact, /(x){0}/ will match the start of any string for any value of x . This regular expression says that we should find x zero times. Well, we can find x zero times between any two characters.

What you want is the start-of-string character, ^ , followed by a negative lookahead assertion, (?!...) . This allows you to match strings that do not begin with a particular sequence of characters.

'http://www.example.com'.gsub(/^(?!http:\/\/)www\./, 'http://www.')
# => 'http://www.example.com'

'www.example.com'.gsub(/^(?!http:\/\/)www\./, 'http://www.')
# => 'http://www.example.com'

Answer 2

Seems like you need to make the capturing group as optional.

> 'http://www.example.com'.gsub(/(http:\/\/)?www\./, 'http://www.')
=> "http://www.example.com"
> 'www.example.com'.gsub(/(http:\/\/)?www\./, 'http://www.')
=> "http://www.example.com"

(http:\\/\\/)? matches the string http:// zero or one times.

OR

You could use negative lookbehind also.

> 'www.example.com'.gsub(/(?<!http:\/\/)www\./, 'http://www.')
=> "http://www.example.com"

Here the substitution should happen because the string www. isn't preceded by http:// .

> 'http://www.example.com'.gsub(/(?<!http:\/\/)www\./, 'http://www.')
=> "http://www.example.com"

Here the substitution won't happen because the string www. is preceded by http:// . So the interpreter returns the original input string without any modifications.