# How do I use a variable in a regexp search string? **URL:** \ **Category:** ruby-talk\ **Created:** [30 January 2010 00:50 UTC](https://rubytalk.org/t/how-do-i-use-a-variable-in-a-regexp-search-string/57197 "2010-01-30T00:50:44Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![joemac](https://avatars.discourse-cdn.com/v4/letter/j/48db29/32.png) [@joemac](https://rubytalk.org/u/joemac)\ **Post date:** [30 January 2010 00:50 UTC](https://rubytalk.org/t/how-do-i-use-a-variable-in-a-regexp-search-string/57197/1 "2010-01-30T00:50:44Z")
Hi All, I dont know how to use a variable in a regex searchstring. ruby seems to only handle hardcoded serachstring in regex's. How do you do this. My code is below. Also - how do you put a variable name in a File.open("filename.txt") statement like this: File.open(filevariable) where filevariable can be set to anything? I could not find this in the docs. Thanks, --Joe ================ snip code =============== #!/usr/bin/ruby F = "filename.txt" searchstring = ARGV[0] print "searchstring is ", searchstring, "\n" File.open("filename.txt").each { |line| # this fails puts line if line =~ searchstring # this fails puts line if line =~ /searchstring/ # this fails puts line if line =~ "searchstring" #this works # puts line } ---
**Author:** ![Harry3](https://avatars.discourse-cdn.com/v4/letter/h/54ee81/32.png) [@Harry3](https://rubytalk.org/u/Harry3)\ **Post date:** [30 January 2010 03:38 UTC](https://rubytalk.org/t/how-do-i-use-a-variable-in-a-regexp-search-string/57197/2 "2010-01-30T03:38:31Z")
> I dont know how to use a variable in a regex searchstring. > ruby seems to only handle hardcoded serachstring in > regex's. How do you do this. My code is below. Does this help? str = "abcdef" searchstring = "cd" p str =~ /#{searchstring}/ Harry ---
**Author:** ![Josh\_Cheek](https://avatars.discourse-cdn.com/v4/letter/j/e79b87/32.png) [@Josh\_Cheek](https://rubytalk.org/u/Josh_Cheek)\ **Post date:** [30 January 2010 16:41 UTC](https://rubytalk.org/t/how-do-i-use-a-variable-in-a-regexp-search-string/57197/3 "2010-01-30T16:41:00Z")
This should work, here is an image showing how to use it: [http://tinypic.com/r/b5pi7m/6](http://tinypic.com/r/b5pi7m/6) filename = 'source.txt' to\_find = Regexp.new ARGV[0] puts "The Regexp is: #{to\_find.inspect}" File.open(filename).each do |line|   if line =~ to\_find     puts "This line matches: #{line}"     puts "This is what matches: #{line[to\_find]}"   end end > **···** > > On Fri, Jan 29, 2010 at 6:50 PM, joemac \ wrote: > > > Hi All, > > > > I dont know how to use a variable in a regex searchstring. > > ruby seems to only handle hardcoded serachstring in > > regex's. How do you do this. My code is below. > > > > Also - how do you put a variable name in a > > File.open("filename.txt") statement like this: > > File.open(filevariable) where filevariable can > > be set to anything? I could not find this in > > the docs. > > > > Thanks, --Joe > > ================ snip code =============== > > #!/usr/bin/ruby > > > > F = "filename.txt" > > searchstring = ARGV[0] > > > > print "searchstring is ", searchstring, "\n" > > > > File.open("filename.txt").each { |line| > > # this fails puts line if line =~ searchstring > > # this fails puts line if line =~ /searchstring/ > > # this fails puts line if line =~ "searchstring" > > > > #this works > > # puts line > > } ---
**Author:** ![7rans](https://avatars.discourse-cdn.com/v4/letter/7/a8b319/32.png) [@7rans](https://rubytalk.org/u/7rans)\ **Post date:** [30 January 2010 03:56 UTC](https://rubytalk.org/t/how-do-i-use-a-variable-in-a-regexp-search-string/57197/4 "2010-01-30T03:56:57Z")
You may need to escape:   p str =~ /#{Regexp.escape(searchstring)}/ > **···** > > On Jan 29, 10:38 pm, Harry Kakueki \ wrote: > > > \> I dont know how to use a variable in a regex searchstring. > > \> ruby seems to only handle hardcoded serachstring in > > \> regex's. How do you do this. My code is below. > > > > Does this help? > > > > str = "abcdef" > > searchstring = "cd" > > p str =~ /#{searchstring}/ ---
**Author:** ![Brian\_Candler](https://avatars.discourse-cdn.com/v4/letter/b/5f9b8f/32.png) [@Brian\_Candler](https://rubytalk.org/u/Brian_Candler)\ **Post date:** [30 January 2010 17:56 UTC](https://rubytalk.org/t/how-do-i-use-a-variable-in-a-regexp-search-string/57197/5 "2010-01-30T17:56:18Z")
Josh Cheek wrote: > File.open(filename).each do |line| >   if line =~ to\_find >     puts "This line matches: #{line}" >     puts "This is what matches: #{line[to\_find]}" or to avoid having to match it twice:   puts "This is what matches: #{$&}" Also:   puts "This is what came before: #{$`}"   puts "This is what came after: #{$'}"   puts "Capture 1 is #{$1}"   # etc > **···** > > -- > Posted via [http://www.ruby-forum.com/\](http://www.ruby-forum.com/%5C). ---
**Author:** ![joemac](https://avatars.discourse-cdn.com/v4/letter/j/48db29/32.png) [@joemac](https://rubytalk.org/u/joemac)\ **Post date:** [22 February 2010 18:45 UTC](https://rubytalk.org/t/how-do-i-use-a-variable-in-a-regexp-search-string/57197/6 "2010-02-22T18:45:17Z")
Hi Folks, Thanks for all the helpful info. It was the #{} syntax that i was missing. This also works for the "filename as a varible" situation when parsing lots of files: filenames = ["input1", "input2", "input3"] for fname in filenames    infile = File.open("#{fname}")    while line = infile.gets()       puts line    end    puts "================== end file ===================" end Very helpfull! Thanks, --JM > **···** > > On Jan 29, 7:38 pm, Harry Kakueki \ wrote: > > > \> I dont know how to use avariablein a regex searchstring. > > \> ruby seems to only handle hardcoded serachstring in > > \> regex's. How do you do this. My code is below. > > > > Does this help? > > > > str = "abcdef" > > searchstring = "cd" > > p str =~ /#{searchstring}/ > > > > Harry ---
**Author:** ![Marnen\_Laibow-Koser](https://avatars.discourse-cdn.com/v4/letter/m/a587f6/32.png) [@Marnen\_Laibow-Koser](https://rubytalk.org/u/Marnen_Laibow-Koser)\ **Post date:** [30 January 2010 13:52 UTC](https://rubytalk.org/t/how-do-i-use-a-variable-in-a-regexp-search-string/57197/7 "2010-01-30T13:52:10Z")
Thomas Sawyer wrote: > **···** > > > On Jan 29, 10:38�pm, Harry Kakueki \ wrote: > > > > > \> I dont know how to use a variable in a regex searchstring. > > > \> ruby seems to only handle hardcoded serachstring in > > > \> regex's. � How do you do this. � My code is below. > > > > > > Does this help? > > > > > > str = "abcdef" > > > searchstring = "cd" > > > p str =~ /#{searchstring}/ > > > > You may need to escape: > > > >   p str =~ /#{Regexp.escape(searchstring)}/ > > But note that #{} only makes sense if you're interpolating a variable > into part of a regex -- say, /January #{year}/. If the variable is the > whole regex, then it's more sensible to do Regex.new(searchstring). > > Best, > -- > Marnen Laibow-Koser > [http://www.marnen.org](http://www.marnen.org) > marnen@marnen.org > -- > Posted via [http://www.ruby-forum.com/\](http://www.ruby-forum.com/%5C). ---
**Author:** ![Rob\_Biedenharn1](https://avatars.discourse-cdn.com/v4/letter/r/57b2e6/32.png) [@Rob\_Biedenharn1](https://rubytalk.org/u/Rob_Biedenharn1)\ **Post date:** [22 February 2010 19:22 UTC](https://rubytalk.org/t/how-do-i-use-a-variable-in-a-regexp-search-string/57197/8 "2010-02-22T19:22:26Z")
In this case, you're already getting strings in fname so "#{fname}" isn't really doing anything useful. -Rob Rob Biedenharn [http://agileconsultingllc.com](http://agileconsultingllc.com) Rob@AgileConsultingLLC.com > **···** > > On Feb 22, 2010, at 1:45 PM, joemac wrote: > > > On Jan 29, 7:38 pm, Harry Kakueki \ wrote: > > > > > > I dont know how to use avariablein a regex searchstring. > > > > ruby seems to only handle hardcoded serachstring in > > > > regex's. How do you do this. My code is below. > > > > > > Does this help? > > > > > > str = "abcdef" > > > searchstring = "cd" > > > p str =~ /#{searchstring}/ > > > > > > Harry > > > > Hi Folks, > > > > Thanks for all the helpful info. > > It was the #{} syntax that i was missing. > > This also works for the "filename as a varible" situation > > when parsing lots of files: > > > > filenames = ["input1", "input2", "input3"] > > > > for fname in filenames > >   infile = File.open("#{fname}") > >   while line = infile.gets() > >      puts line > >   end > >   puts "================== end file ===================" > > end > > > > Very helpfull! > > > > Thanks, --JM