# Re: Easy way to check is sub!() succeeds?

**URL:** <https://www.ruby-forum.com/t/re-easy-way-to-check-is-sub-succeeds/73045>\
**Category:** Ruby\
**Created:** [October 20, 2006, 9:03pm UTC](https://www.ruby-forum.com/t/re-easy-way-to-check-is-sub-succeeds/73045 "2006-10-20T21:03:15Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![phrogz](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/2acd7d/32.png) [@phrogz](https://www.ruby-forum.com/u/phrogz)\
**Post date:** [October 20, 2006, 9:03pm UTC](https://www.ruby-forum.com/t/re-easy-way-to-check-is-sub-succeeds/73045/1 "2006-10-20T21:03:15Z")

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> But sub! always returns nil, even when it succeeds (in Perl, it will  
> return the number of substitutions made, enabling “next if  
> s/.$suf$//”)

Er, no it doesn’t.

irb(main):001:0\> s=“aaabbb”  
=\> “aaabbb”  
irb(main):002:0\> s.sub!(/a+/,’’)  
=\> “bbb”  
irb(main):003:0\> s.sub!(/zzz/,’’)  
=\> nil

C:\>ri String.sub!  
------------------------------------------------------------ String#sub!  
str.sub!(pattern, replacement) =\> str or nil  
str.sub!(pattern) {|match| block } =\> str or nil

---

<div class="post-metadata">

**Author:** ![phrogz](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/2acd7d/32.png) [@phrogz](https://www.ruby-forum.com/u/phrogz)\
**Post date:** [October 20, 2006, 11:57pm UTC](https://www.ruby-forum.com/t/re-easy-way-to-check-is-sub-succeeds/73045/2 "2006-10-20T23:57:44Z")

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Gavin K. wrote:

> irb(main):003:0\> s.sub!(/zzz/,’’)  
> =\> nil

Ok, that makes more sense. Turns out I was upsetting Ruby’s  
scoping/aliasing. Eventually I got an error “Can’t modify frozen  
string”, which tipped me off. Now I have:

```
ARGV.each do |file|
   path = file.dup # copy to tweak
   zip.each do |suf, cmd|
      if path.sub!(%r{\.#{suf}$}, '')

```

And everything works ok. Seems like a loop w/i a loop thing? Anyways,  
thanks for the sub!() ptr.

-Nate