Ruby gsub替换中的行为不一致？

Question

The two gsub's yield different outcomes. Can anybody explain why?

Code is also available at https://gist.github.com/franklsf95/6c0f8938f28706b5644d .

    ver = 9999
    str = "\t<key>CFBundleDevelopmentRegion</key>\n\t<string>en</string>\n\t<key>CFBundleVersion</key>\n\t<string>0.1.190</string>\n\t<key>AppID</key>\n\t<string>000000000000000</string>"
    puts str.gsub /(CFBundleVersion<\/key>\n\t.*\.).*(<\/string>)/, "#{$1}#{ver}#{$2}"
    puts '--------'
    puts str.gsub /(CFBundleVersion<\/key>\n\t.*\.).*(<\/string>)/, "#{$1}#{ver}#{$2}"

My ruby version is ruby 2.1.2p95 (2014-05-08 revision 45877) [x86_64-darwin13.0] (MRI). On my machine, the outcome is:

<key>CFBundleDevelopmentRegion</key>
<string>en</string>
<key>9999
<key>AppID</key>
<string>000000000000000</string>
--------
<key>CFBundleDevelopmentRegion</key>
<string>en</string>
<key>CFBundleVersion</key>
<string>0.1.9999</string>
<key>AppID</key>
<string>000000000000000</string>

The second one is the desired effect, but the first one is wrong.

Answer 1

It has to do with timing and how ruby regexes work.

gsub sets $1 and $2 , but not until after it completes. So when you run the first time through, they're blank. When you run the second time, they were set by the previous gsub . If you want to do regex captures in place, you need \\1 and \\2 , like this:

puts str.gsub /(CFBundleVersion<\/key>\n\t.*\.).*(<\/string>)/, '\1' + ver.to_s + '\2'

Answer 2

If you use the block form of gsub(), your code will work correctly:

ver = 9999

str = "\t<key>CFBundleDevelopmentRegion</key>\n\t<string>en</string>\n\t<key>CFBundleVersion</key>\n\t<string>0.1.190</string>\n\t<key>AppID</key>\n\t<string>000000000000000</string>"

puts str.gsub(/(CFBundleVersion<\/key>\n\t.*\.).*(<\/string>)/) {|match|
  "#{$1}#{ver}#{$2}"
}

puts '-' * 20

puts str.gsub(/(CFBundleVersion<\/key>\n\t.*\.).*(<\/string>)/) {|match|
  "#{$1}#{ver}#{$2}"
}

--output:--
    <key>CFBundleDevelopmentRegion</key>
    <string>en</string>
    <key>CFBundleVersion</key>
    <string>0.1.9999</string>
    <key>AppID</key>
    <string>000000000000000</string>
--------------------
    <key>CFBundleDevelopmentRegion</key>
    <string>en</string>
    <key>CFBundleVersion</key>
    <string>0.1.9999</string>
    <key>AppID</key>
    <string>000000000000000</string>

The docs describe this behavior:

If replacement is a String, ... However, within replacement the special match variables, such as $&, will not refer to the current match.

...

In the block form, the current match string is passed in as a parameter, and variables such as $1, $2, $`, $&, and $' will be set appropriately. The value returned by the block will be substituted for the match on each call.