# \[Q\] specify start postion of Regexp matching **URL:** \ **Category:** ruby-talk\ **Created:** [25 November 2007 15:20 UTC](https://rubytalk.org/t/q-specify-start-postion-of-regexp-matching/42487 "2007-11-25T15:20:25Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![Makoto\_Kuwata](https://avatars.discourse-cdn.com/v4/letter/m/a4c791/32.png) [@Makoto\_Kuwata](https://rubytalk.org/u/Makoto_Kuwata)\ **Post date:** [25 November 2007 15:20 UTC](https://rubytalk.org/t/q-specify-start-postion-of-regexp-matching/42487/1 "2007-11-25T15:20:25Z")
Hi, all. Is it possible to specify start position of Regexp matching?     str = "foo bar baz"     m = /ba/.match(str)     p m.begin(0) #=\> 4     m = /ba/.match(str, 5) # is it possible?     p m.begin(0) #=\> 8 (if possible) If it is possible, some kind of parser or scanner can be implemented easily. # StringScanner is a litte too big, I think. > **···** > > -- > makoto kuwata ---
**Author:** ![Eric\_I2](https://avatars.discourse-cdn.com/v4/letter/e/22d042/32.png) [@Eric\_I2](https://rubytalk.org/u/Eric_I2)\ **Post date:** [25 November 2007 15:39 UTC](https://rubytalk.org/t/q-specify-start-postion-of-regexp-matching/42487/2 "2007-11-25T15:39:59Z")
You could try something like this:     m = /^.{5,}(ba)/.match(str)     p m.begin(1) In the regular expression, you're saying start at the beginning and skip at least 5 characters. But then we have to use parens to "note" the part you're interested in, and then we have to pass 1 rather than 0 to begin, so it reports the location of the first noted match (0 would report where the entire Regexp matched, and that would be the beginning of the line). An alternative would be to slice the first n characters off the front of the string and then do the match. Eric > **···** > > On Nov 25, 10:18 am, makoto kuwata \ wrote: > > > Hi, all. > > > > Is it possible to specify start position of Regexp matching? > > > >     str = "foo bar baz" > >     m = /ba/.match(str) > >     p m.begin(0) #=\> 4 > >     m = /ba/.match(str, 5) # is it possible? > >     p m.begin(0) #=\> 8 (if possible) > > > > If it is possible, some kind of parser or scanner can be > > implemented easily. > > # StringScanner is a litte too big, I think. > > ==== > > Interested in hands-on, on-site Ruby training? See [http://LearnRuby.com](http://LearnRuby.com) > for information about a well-reviewed class. ---
**Author:** ![Axel\_Etzold](https://avatars.discourse-cdn.com/v4/letter/a/ba8739/32.png) [@Axel\_Etzold](https://rubytalk.org/u/Axel_Etzold)\ **Post date:** [25 November 2007 16:46 UTC](https://rubytalk.org/t/q-specify-start-postion-of-regexp-matching/42487/3 "2007-11-25T16:46:39Z")
-------- Original-Nachricht -------- > Datum: Mon, 26 Nov 2007 00:20:25 +0900 > Von: makoto kuwata \ > An: ruby-talk@ruby-lang.org > Betreff: [Q] specify start postion of Regexp matching > Hi, all. > > Is it possible to specify start position of Regexp matching? > >     str = "foo bar baz" >     m = /ba/.match(str) >     p m.begin(0) #=\> 4 >     m = /ba/.match(str, 5) # is it possible? >     p m.begin(0) #=\> 8 (if possible) > > If it is possible, some kind of parser or scanner can be > implemented easily. > # StringScanner is a litte too big, I think. > > -- > makoto kuwata Dear Makoto, what about : class Regexp   def match\_index\_offset(string,start\_pos)     temp=string[start\_pos..-1]     ref=self.match(temp)     return temp.index(ref[0])+start\_pos   end end str = "foo bar baz" m = /ba/.match\_index\_offset(str,5) p m Best regards, Axel > **···** > > -- > Psssst! Schon vom neuen GMX MultiMessenger gehört? > Der kann`s mit allen: [http://www.gmx.net/de/go/multimessenger](http://www.gmx.net/de/go/multimessenger) ---
**Author:** ![Yukihiro\_Matsumoto2](https://avatars.discourse-cdn.com/v4/letter/y/b9e5f3/32.png) [@Yukihiro\_Matsumoto2](https://rubytalk.org/u/Yukihiro_Matsumoto2)\ **Post date:** [25 November 2007 23:24 UTC](https://rubytalk.org/t/q-specify-start-postion-of-regexp-matching/42487/4 "2007-11-25T23:24:04Z")
Hi, > **···** > > In message "Re: [Q] specify start postion of Regexp matching" on Mon, 26 Nov 2007 00:20:25 +0900, makoto kuwata \ writes: > > > Is it possible to specify start position of Regexp matching? > > > >    str = "foo bar baz" > >    m = /ba/.match(str) > >    p m.begin(0) #=\> 4 > >    m = /ba/.match(str, 5) # is it possible? > >    p m.begin(0) #=\> 8 (if possible) > > str.index(/ba/, 5) ? > >               matz. ---
**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:** [25 November 2007 16:25 UTC](https://rubytalk.org/t/q-specify-start-postion-of-regexp-matching/42487/5 "2007-11-25T16:25:05Z")
Another alternative is to use String#scan - we would have to know what the OP really wants to parse though to decide whether it's a feasible solution. Kind regards   robert > **···** > > On 25.11.2007 16:39, Eric I. wrote: > > > On Nov 25, 10:18 am, makoto kuwata \ wrote: > > > > > Hi, all. > > > > > > Is it possible to specify start position of Regexp matching? > > > > > >     str = "foo bar baz" > > >     m = /ba/.match(str) > > >     p m.begin(0) #=\> 4 > > >     m = /ba/.match(str, 5) # is it possible? > > >     p m.begin(0) #=\> 8 (if possible) > > > > > > If it is possible, some kind of parser or scanner can be > > > implemented easily. > > > # StringScanner is a litte too big, I think. > > > > You could try something like this: > > > >     m = /^.{5,}(ba)/.match(str) > >     p m.begin(1) > > > > In the regular expression, you're saying start at the beginning and > > skip at least 5 characters. But then we have to use parens to "note" > > the part you're interested in, and then we have to pass 1 rather than > > 0 to begin, so it reports the location of the first noted match (0 > > would report where the entire Regexp matched, and that would be the > > beginning of the line). > > > > An alternative would be to slice the first n characters off the front > > of the string and then do the match. ---
**Author:** ![Makoto\_Kuwata](https://avatars.discourse-cdn.com/v4/letter/m/a4c791/32.png) [@Makoto\_Kuwata](https://rubytalk.org/u/Makoto_Kuwata)\ **Post date:** [25 November 2007 18:00 UTC](https://rubytalk.org/t/q-specify-start-postion-of-regexp-matching/42487/6 "2007-11-25T18:00:02Z")
Thank you, all. Eric l wrote: > You could try something like this: >     m = /^.{5,}(ba)/.match(str) >     p m.begin(1) In my program, start position is variable such as      def f(n)        m = /^.{n,}(ba)/.match(str)        ...      end In this case, /^.{n,}(ba)/ is created for each time. It is not effective. Robert Klemme wrote: > Another alternative is to use String#scan - String#scan is useful only when regexp pattern is fixed.     input.scan(/FIXED-REGEXP/) do ... end Using String#scan, it is not able to change regexp pattern in the loop. Axel Etzold wrote: >                temp=string[start\_pos..-1] >                ref=self.match(temp) >                return temp.index(ref[0])+start\_pos In this solution, temp substring is created every time. If input string is long, it is not efficient. Thanks to all your advices. I'm going to propose to support start position in Regexp#match(). > **···** > > -- > makoto kuwata ---
**Author:** ![Makoto\_Kuwata](https://avatars.discourse-cdn.com/v4/letter/m/a4c791/32.png) [@Makoto\_Kuwata](https://rubytalk.org/u/Makoto_Kuwata)\ **Post date:** [25 November 2007 23:49 UTC](https://rubytalk.org/t/q-specify-start-postion-of-regexp-matching/42487/7 "2007-11-25T23:49:59Z")
No, String#index returns Fixnum (position), but I want MatchData. Regexp#match(string, start=0) in Ruby1.9 is the best solution I want. Is there any plan to implement it into Ruby1.8? > **···** > > Yukihiro Matsumoto \ wrote: > > > str.index(/ba/, 5) ? > > -- > makoto kuwata ---
**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:** [25 November 2007 19:20 UTC](https://rubytalk.org/t/q-specify-start-postion-of-regexp-matching/42487/8 "2007-11-25T19:20:00Z")
> Robert Klemme wrote: > > > Another alternative is to use String#scan - > > String#scan is useful only when regexp pattern is fixed. >     input.scan(/FIXED-REGEXP/) do ... end > Using String#scan, it is not able to change regexp pattern > in the loop. But in various situations it is possible to use a unified regexp for scanning or a regexp that comprises all other patterns. > Axel Etzold wrote: > > >                temp=string[start\_pos..-1] > >                ref=self.match(temp) > >                return temp.index(ref[0])+start\_pos > > In this solution, temp substring is created every time. > If input string is long, it is not efficient. This is not true. Creating a substring is fairly cheap because the character buffer is not copied (copy on write). > I'm going to propose to support start position in Regexp#match(). For the time being it's faster to use one of the other alternatives. Also, with the new regexp engine in 1.9 your feature might be present already. Kind regards   robert > **···** > > On 25.11.2007 18:58, makoto kuwata wrote: ---
**Author:** ![Makoto\_Kuwata](https://avatars.discourse-cdn.com/v4/letter/m/a4c791/32.png) [@Makoto\_Kuwata](https://rubytalk.org/u/Makoto_Kuwata)\ **Post date:** [25 November 2007 23:50 UTC](https://rubytalk.org/t/q-specify-start-postion-of-regexp-matching/42487/9 "2007-11-25T23:50:00Z")
I found that it is able to get MatchData by Regexp.last\_match() after String#index(). Well, I think Regexp#match(string, start=0) is the natural way, but String#index(regexp, start) can be the good solution. Thank you, Matz. > **···** > > makoto kuwata \ wrote: > > > \> str.index(/ba/, 5) ? > > > > No, String#index returns Fixnum (position), but I want MatchData. > > -- > makoto kuwata ---
**Author:** ![Makoto\_Kuwata](https://avatars.discourse-cdn.com/v4/letter/m/a4c791/32.png) [@Makoto\_Kuwata](https://rubytalk.org/u/Makoto_Kuwata)\ **Post date:** [25 November 2007 23:25 UTC](https://rubytalk.org/t/q-specify-start-postion-of-regexp-matching/42487/10 "2007-11-25T23:25:05Z")
> \> In this solution, temp substring is created every time. > \> If input string is long, it is not efficient. > > This is not true. Creating a substring is fairly cheap because the > character buffer is not copied (copy on write). You are right. If input string is not modified, creating substring doesn't copy anything. Creating substring may be the solution I wanted. > \> I'm going to propose to support start position in Regexp#match(). > > For the time being it's faster to use one of the other alternatives. > Also, with the new regexp engine in 1.9 your feature might be present > already. I found that Regexp#match() can take optional 2nd argument which specifies matching start position in Ruby1.9. Good news. Thank you, Robert. > **···** > > Robert Klemme \ wrote: > > -- > makoto kuwata ---
**Author:** ![Jordan\_Callicoat](https://avatars.discourse-cdn.com/v4/letter/j/9de053/32.png) [@Jordan\_Callicoat](https://rubytalk.org/u/Jordan_Callicoat)\ **Post date:** [26 November 2007 05:20 UTC](https://rubytalk.org/t/q-specify-start-postion-of-regexp-matching/42487/11 "2007-11-26T05:20:10Z")
What's the difference between 1.9 Regexp#match(string, start=n) and 1.8 Regexp#match(string[n..-1])?? You have to create a sub-string with the 1.8 version, but according to Robert Klemme (above) it's just creating a pointer into the original string if you're not changing the substring or original string. Besides, even if you did get a copy, it's anonymous and should be garbage collected soon. If I understand everything correctly, the 1.9 version would just basically be a convenience feature over the 1.8 way? $ irb19 irb(main):001:0\> RUBY\_VERSION =\> "1.9.0" irb(main):002:0\> m = /oo/.match("foo", start=1) =\> #\ irb(main):003:0\> m[0] =\> "oo" $ irb irb(main):001:0\> RUBY\_VERSION =\> "1.8.6" irb(main):002:0\> m = /oo/.match("foo"[1..-1]) =\> #\ irb(main):003:0\> m[0] =\> "oo" Regards, Jordan ---
**Author:** ![7stud](https://avatars.discourse-cdn.com/v4/letter/7/57b2e6/32.png) [@7stud](https://rubytalk.org/u/7stud)\ **Post date:** [26 November 2007 06:07 UTC](https://rubytalk.org/t/q-specify-start-postion-of-regexp-matching/42487/12 "2007-11-26T06:07:40Z")
Jordan Callicoat wrote: > You have to create a sub-string with > the 1.8 version, but according to Robert Klemme (above) it's just > creating a pointer into the original string if you're not changing the > substring or original string. I'm having a hard time confirming that: str = "hello" sub\_str = str[1, 2] puts str.object\_id --\>76750 puts sub\_str.object\_id --\>76740 puts sub\_str.class --\>String > **···** > > -- > Posted via [http://www.ruby-forum.com/\](http://www.ruby-forum.com/%5C). ---
**Author:** ![Jordan\_Callicoat](https://avatars.discourse-cdn.com/v4/letter/j/9de053/32.png) [@Jordan\_Callicoat](https://rubytalk.org/u/Jordan_Callicoat)\ **Post date:** [26 November 2007 06:50 UTC](https://rubytalk.org/t/q-specify-start-postion-of-regexp-matching/42487/13 "2007-11-26T06:50:00Z")
I'm not sure how to confirm it, other than just looking at the source, and since I'm very poor at C programming, it probably wouldn't help for me to try that. I'm sure Robert can demonstrate. But I will say that I'm not suprised that they have different object\_id, because they are different objects. The copy on write is just a back-end optimization where you pretend that two objects that point to the same data are unique copies in the front-end, but you don't actually move any data in the back-end until you have to (i,e., when one of the objects is changed). Regards, Jordan > **···** > > On Nov 26, 12:07 am, 7stud -- \ wrote: > > > Jordan Callicoat wrote: > > \> You have to create a sub-string with > > \> the 1.8 version, but according to Robert Klemme (above) it's just > > \> creating a pointer into the original string if you're not changing the > > \> substring or original string. > > > > I'm having a hard time confirming that: ---
**Author:** ![George3](https://avatars.discourse-cdn.com/v4/letter/g/51bf81/32.png) [@George3](https://rubytalk.org/u/George3)\ **Post date:** [27 November 2007 05:07 UTC](https://rubytalk.org/t/q-specify-start-postion-of-regexp-matching/42487/14 "2007-11-27T05:07:39Z")
A new ruby object is created, but the string buffer that it points to is only copied on write. > **···** > > On Nov 26, 2007 5:07 PM, 7stud -- \ wrote: > > > Jordan Callicoat wrote: > > \> You have to create a sub-string with > > \> the 1.8 version, but according to Robert Klemme (above) it's just > > \> creating a pointer into the original string if you're not changing the > > \> substring or original string. > > > > I'm having a hard time confirming that: > > > > str = "hello" > > sub\_str = str[1, 2] > > > > puts str.object\_id > > --\>76750 > > > > puts sub\_str.object\_id > > --\>76740 > > > > puts sub\_str.class > > --\>String ---
**Author:** ![Jordan\_Callicoat](https://avatars.discourse-cdn.com/v4/letter/j/9de053/32.png) [@Jordan\_Callicoat](https://rubytalk.org/u/Jordan_Callicoat)\ **Post date:** [26 November 2007 07:10 UTC](https://rubytalk.org/t/q-specify-start-postion-of-regexp-matching/42487/15 "2007-11-26T07:10:00Z")
Well, I did anyhow... [http://svn.ruby-lang.org/repos/ruby/branches/ruby\_1\_8/ruby.h](http://svn.ruby-lang.org/repos/ruby/branches/ruby_1_8/ruby.h) [http://svn.ruby-lang.org/repos/ruby/branches/ruby\_1\_8/string.c](http://svn.ruby-lang.org/repos/ruby/branches/ruby_1_8/string.c) And I think the functions of interest are str\_new3 and str\_new4 (called from rb\_str\_substr). Specifically, the assignment of RSTRING(str2)-\>aux.shared. But like I said, I'm not great with C, so I could be mistaken. Regards, Jordan > **···** > > On Nov 26, 12:45 am, MonkeeSage \ wrote: > > > I'm not sure how to confirm it, other than just looking at the source, > > and since I'm very poor at C programming, it probably wouldn't help > > for me to try that. ---
**Author:** ![Jordan\_Callicoat](https://avatars.discourse-cdn.com/v4/letter/j/9de053/32.png) [@Jordan\_Callicoat](https://rubytalk.org/u/Jordan_Callicoat)\ **Post date:** [26 November 2007 09:05 UTC](https://rubytalk.org/t/q-specify-start-postion-of-regexp-matching/42487/16 "2007-11-26T09:05:04Z")
Here's a test to show that my reading of the source, and Robert's assertion, is correct (there is probably a better way to do this...): #!/usr/bin/env ruby # disable GC to get fair reading of actual allocation cost GC.disable def free\_megs    (`free -o`.split("\n")[1].split(' ')[3].to\_i/1024).to\_s end puts "Free megabytes " + free\_megs # make a one megabyte string s1 = "a" \* 1048576 s100 = "" # placeholder to be filled in below # make 100 substrings of it 0.upto(101) { |i| eval("s#{i}=s1[0..-1]") } puts s100.length.to\_s puts "Free megabytes " + free\_megs Output: Free megabytes 588 1048576 Free megabytes 587 Only one meg is used, which is the length of the original string. So, by inductive inference, the substrings are only pointers back to the original string rather than copies of the data. Regards, Jordan