# Sub/gsub bug? **URL:** \ **Category:** ruby-talk\ **Created:** [18 May 2008 04:10 UTC](https://rubytalk.org/t/sub-gsub-bug/46568 "2008-05-18T04:10:19Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Matt12](https://avatars.discourse-cdn.com/v4/letter/m/5e9695/32.png) [@Matt12](https://rubytalk.org/u/Matt12)\ **Post date:** [18 May 2008 04:10 UTC](https://rubytalk.org/t/sub-gsub-bug/46568/1 "2008-05-18T04:10:19Z")
Is this a bug? irb(main):032:0\> "matt's test".sub(/'/, "\\'") =\> "matts tests test" It's trying to replace a single quote with an escaped quote. Thanks in advance, Matt ---
**Author:** ![Matt12](https://avatars.discourse-cdn.com/v4/letter/m/5e9695/32.png) [@Matt12](https://rubytalk.org/u/Matt12)\ **Post date:** [18 May 2008 05:50 UTC](https://rubytalk.org/t/sub-gsub-bug/46568/2 "2008-05-18T05:50:13Z")
Phlip wrote: > Untill someone dissects the source and finds (or adds) a better fix, here's > a workaround: Thanks for the reply, Phlip. As it turns out, when I was researching this problem I found out that you can use replaceable parameters in DBI queries instead of worrying about escaping quotes, so that's a relief. I was just kind of curious why gsub() wasn't working the way I thought it would. > irb(main):012:0\> "matt's test".sub(/'/){"\\'"} > =\> "matt\\'s test" > > That surprised me too, because I thought that sub's block form also > interpolated \n group inserters, but I suppose that blocks have room for > true $1 match references. \1 is often available in some regular expressions > that cram all the searchers and replacers into one huge line, so a $1 is > naturally not available inside its own source expression. (I also don't > happen to know if Ruby supports such super-regices!) Yeah that's strange that they would write two replacement string parsers, but I guess there's probably a good reason for it. Super-regices? You mean like s/xxx/yyy/g? I haven't seen that anywhere in my Ruby travels, but I'm kind of new. Matt ---
**Author:** ![J\_Cooper](https://avatars.discourse-cdn.com/v4/letter/j/3e96dc/32.png) [@J\_Cooper](https://rubytalk.org/u/J_Cooper)\ **Post date:** [18 May 2008 06:13 UTC](https://rubytalk.org/t/sub-gsub-bug/46568/3 "2008-05-18T06:13:53Z")
Matt Gregory wrote: > Is this a bug? > > irb(main):032:0\> "matt's test".sub(/'/, "\\'") > =\> "matts tests test" > > It's trying to replace a single quote with an escaped quote. > > Thanks in advance, > Matt irb(main):002:0\> puts "matt's test".sub(/'/, "\\\\'") matt\'s test =\> nil The gsub replacement string is a crazy thing when it comes to escaping, because remember you can put backreferences in it. Any double quoted string needs 2 backslashes to represent a literal backslash, but remember in this context, a literal backslash represents the backreference syntax, so you need to escape it with another "literal backslash" of 2 backslashes. At least, I think that's how I understand it 馃槢 > **路路路** > > -- > Posted via [http://www.ruby-forum.com/\](http://www.ruby-forum.com/%5C). ---
**Author:** ![Robert\_K1](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/robert_k1/32/1830_2.png) [@Robert\_K1](https://rubytalk.org/u/Robert_K1)\ **Post date:** [18 May 2008 11:40 UTC](https://rubytalk.org/t/sub-gsub-bug/46568/4 "2008-05-18T11:40:12Z")
> Matt wrote: > > > Is this a bug? > > > > irb(main):032:0\> "matt's test".sub(/'/, "\\'") > > =\> "matts tests test" > > > > It's trying to replace a single quote with an escaped quote. > > To understand the bug, try this: There is no bug. > irb(main):009:0\> "matt's test".sub(/(')/, '\1') > =\> "matt's test" > > The 'single ticks' interpret \1 as a literal \ and a 1, not an escape. Then .sub sees the \1, and replaces it with the 1nth (grouping) from the Regexp - which is a literal tick '. > > Now try it with "double quotes": > > irb(main):005:0\> "matt's test".sub(/(')/, "\\1") > =\> "matt's test" > > The " forces us to escape the \ as \\ to get a literal \. And a literal \ in a replacement string is a meta character. To get a literal in a replacement, you need \\ in the string and consequently \\\\ in a double quoted string. > So your \\' indeed goes in as a literal \'. But the group replacer still sees a \, and snarfs it, expecting a number after it. I would call that a bug (essentially because I might personally be capable of correctly parsing a \1!) No, this is not a bug and it is not the "group replacer". The replacement string is parsed by the regexp engine and since the backslash does not escape a character that has meta capabilities (such as & and 1..9) it is silently discarded. > Untill someone dissects the source and finds (or adds) a better fix, here's a workaround: > > irb(main):012:0\> "matt's test".sub(/'/){"\\'"} > =\> "matt\\'s test" No, the proper way to do it is this: irb(main):002:0\> "matt's test".sub(/'/, "\\\\'") =\> "matt\\'s test" irb(main):003:0\> puts "matt's test".sub(/'/, "\\\\'") matt\'s test =\> nil irb(main):006:0\> "matt's test".sub(/'/, '\\\\\'') =\> "matt\\'s test" irb(main):007:0\> puts "matt's test".sub(/'/, '\\\\\'') matt\'s test =\> nil Blocks are only needed if you need to do some calculations based on the match, e.g. irb(main):008:0\> "There is 1 number in this string".gsub(/\d+/) {|m| m.to\_i \* 34} =\> "There is 34 number in this string" > That surprised me too, because I thought that sub's block form also interpolated \n group inserters, but I suppose that blocks have room for true $1 match references. Yes. > \1 is often available in some regular expressions that cram all the searchers and replacers into one huge line, so a $1 is naturally not available inside its own source expression. (I also don't happen to know if Ruby supports such super-regices!) I am not sure what you mean here. The reason $1 cannot be used in regular expressions and replacement strings is that it is a variable that gets its value before the regexp is created (of course you can use it but it cannot refer to matching of the regexp you put it into). You can use groups inside the regular expression as well as in the replacement string if this is what you mean. irb(main):017:0\> "aba abc".gsub(/(.)(b)\1/, "[\\1]\<\\2\>[\\1]") =\> "[a]\[a] abc" Kind regards   robert > **路路路** > > On 18.05.2008 06:33, Phlip wrote: ---
**Author:** ![Matt12](https://avatars.discourse-cdn.com/v4/letter/m/5e9695/32.png) [@Matt12](https://rubytalk.org/u/Matt12)\ **Post date:** [18 May 2008 07:55 UTC](https://rubytalk.org/t/sub-gsub-bug/46568/5 "2008-05-18T07:55:07Z")
J. Cooper wrote: > The gsub replacement string is a crazy thing when it comes to escaping, > because remember you can put backreferences in it. Any double quoted > string needs 2 backslashes to represent a literal backslash, but > remember in this context, a literal backslash represents the > backreference syntax, so you need to escape it with another "literal > backslash" of 2 backslashes. > > At least, I think that's how I understand it 馃槢 Yeah, I get all that, but I didn't think that \' was a backreference. I thought it was limited to numbers and the ampersand. But now that I think about it, Perl has a $' variable that contains the portion of the string after the match, so I guess that's where the idea came from. I've never seen that in backreference form before. irb(main):005:0\> "matt's test".gsub(/'/, "\\`") =\> "mattmatts test" irb(main):006:0\> "matt's test".gsub(/'/, "\\&") =\> "matt's test" irb(main):007:0\> "matt's test".gsub(/'/, "\\'") =\> "matts tests test" Yay! Matt ---
**Author:** ![Robert\_K1](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/robert_k1/32/1830_2.png) [@Robert\_K1](https://rubytalk.org/u/Robert_K1)\ **Post date:** [18 May 2008 11:50 UTC](https://rubytalk.org/t/sub-gsub-bug/46568/6 "2008-05-18T11:50:06Z")
^^^^^^^^^^^^^^^^^^ This was wrong of course as Matt demonstrated irb(main):019:0\> "abc".gsub(/b/, "\<\\'\>") =\> "a\c" But the backslash is just retained if it does not appear with a meta capable character: irb(main):020:0\> "abc".gsub(/b/, "\<\\a\>") =\> "a\<\\a\>c" irb(main):021:0\> puts "abc".gsub(/b/, "\<\\a\>") a\<\a\>c =\> nil Sorry, for the added confusion. Kind regards   robert > **路路路** > > On 18.05.2008 13:38, Robert Klemme wrote: > > > On 18.05.2008 06:33, Phlip wrote: > > > > > Matt wrote: > > > > > > > Is this a bug? > > > > > > > > irb(main):032:0\> "matt's test".sub(/'/, "\\'") > > > > =\> "matts tests test" > > > > > > > > It's trying to replace a single quote with an escaped quote. > > > > > > To understand the bug, try this: > > > > There is no bug. > > > > > irb(main):009:0\> "matt's test".sub(/(')/, '\1') > > > =\> "matt's test" > > > > > > The 'single ticks' interpret \1 as a literal \ and a 1, not an escape. Then .sub sees the \1, and replaces it with the 1nth (grouping) from the Regexp - which is a literal tick '. > > > > > > Now try it with "double quotes": > > > > > > irb(main):005:0\> "matt's test".sub(/(')/, "\\1") > > > =\> "matt's test" > > > > > > The " forces us to escape the \ as \\ to get a literal \. > > > > And a literal \ in a replacement string is a meta character. To get a literal in a replacement, you need \\ in the string and consequently \\\\ in a double quoted string. > > > > > So your \\' indeed goes in as a literal \'. But the group replacer still sees a \, and snarfs it, expecting a number after it. I would call that a bug (essentially because I might personally be capable of correctly parsing a \1!) > > > > No, this is not a bug and it is not the "group replacer". The replacement string is parsed by the regexp engine and since the backslash does not escape a character that has meta capabilities (such as & and 1..9) it is silently discarded.