# Non-greediness in a regex - need some help verifying syntax **URL:** \ **Category:** Ruby\ **Created:** [August 3, 2006, 11:31pm UTC](https://www.ruby-forum.com/t/non-greediness-in-a-regex-need-some-help-verifying-syntax/101276 "2006-08-03T23:31:05Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![weyus](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7ab992/32.png) [@weyus](https://www.ruby-forum.com/u/weyus)\ **Post date:** [August 3, 2006, 11:31pm UTC](https://www.ruby-forum.com/t/non-greediness-in-a-regex-need-some-help-verifying-syntax/101276/1 "2006-08-03T23:31:05Z")
All, Need some medium - level regex help. Here’s my regex: /~^LNK:[\t\r\n]+?^~/m I’m trying to find all occurrences of strings in my big string that are between ~^LNK: and ^~ sequences of characters that have at least one tab, form feed, or newline character between those two characters. I use the multiline option so that I can match on the newlines. What I’m seeing is the string that is consumed by this regex spans many many many ~^LNK ^~ pairs so that I am removing a bunch of tabs, newlines, etc. that I don’t want to. I understand the concept of greediness in regexes, so I put the ? after the [\t\r\n] sequence. Why is the match spanning so many pairs of the delimiter sequences? Why doesn’t regex engine stop attempting to match when it sees that first ^~ after the ~^LNK:? Any help is appreciated. Thanks, Wes ---
**Author:** ![weyus](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7ab992/32.png) [@weyus](https://www.ruby-forum.com/u/weyus)\ **Post date:** [August 3, 2006, 11:45pm UTC](https://www.ruby-forum.com/t/non-greediness-in-a-regex-need-some-help-verifying-syntax/101276/2 "2006-08-03T23:45:39Z")
On 8/3/06, Wes G. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > All, > > Need some medium - level regex help. > > Here’s my regex: /~^LNK:[\t\r\n]+?^~/m Hmmm I fail to reproduce the problem is there nothing missing between “[\t\r\n]+?” and “^” ? And if so the missing link is probably what’s consuming all your data. I’m trying to find all occurrences of strings in my big string that are > between > ~^LNK: and ^~ sequences of characters that have at least one tab, form > feed, or newline character between those two characters. and something else as I said above, no? Robert – Deux choses sont infinies : l’univers et la bêtise humaine ; en ce qui concerne l’univers, je n’en ai pas acquis la certitude absolue. - Albert Einstein ---
**Author:** ![weyus](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7ab992/32.png) [@weyus](https://www.ruby-forum.com/u/weyus)\ **Post date:** [August 4, 2006, 12:09am UTC](https://www.ruby-forum.com/t/non-greediness-in-a-regex-need-some-help-verifying-syntax/101276/3 "2006-08-04T00:09:45Z")
I realized I made an error when I did the original post. Now my problem is that it won’t find any of these occurrences now. So @bigstring.scan(/~^LNK:[\t\r\n]+?^~/m) isn’t returning anything. My guess is because there are no occurrences of a tab, newline, or line feed character _immediately_ after ~^LNK. If I do /~^LNK:._?[\t\r\n]+?._?^~/m - that should pick up what I want, correct? Wes ---
**Author:** ![weyus](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7ab992/32.png) [@weyus](https://www.ruby-forum.com/u/weyus)\ **Post date:** [August 4, 2006, 12:53am UTC](https://www.ruby-forum.com/t/non-greediness-in-a-regex-need-some-help-verifying-syntax/101276/4 "2006-08-04T00:53:22Z")
I’m not exactly an expert on regexs, to say the least, but I think .\*? always matches an empty string and is therefore useless. I would try something like @bigstring.scan(/~^LNK:[^\t\r\n]_[\t\r\n]+?[^\t\r\n]_^~/m) I have not test this. Regards, Morton ---
**Author:** ![weyus](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7ab992/32.png) [@weyus](https://www.ruby-forum.com/u/weyus)\ **Post date:** [August 4, 2006, 1:02am UTC](https://www.ruby-forum.com/t/non-greediness-in-a-regex-need-some-help-verifying-syntax/101276/5 "2006-08-04T01:02:55Z")
On Aug 3, 2006, at 6:09 PM, Wes G. wrote: > Wes > > –Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/). Morton G. wrote: > I’m not exactly an expert on regexs, to say the least, but I think .\*? > always matches an empty string and is therefore useless. I would try > something like > > @bigstring.scan(/~^LNK:[^\t\r\n]_[\t\r\n]+?[^\t\r\n]_^~/m) > > I have not test this. > > Regards, Morton No, that’s not true. .\*? It will match whatever it needs to get to the next item: irb(main):001:0\> “asidjoaisdj”.match(/._?j/)[0] =\> “asidj” irb(main):002:0\> “asidjoaisdj”.match(/._?d/)[0] =\> “asid” irb(main):003:0\> “asidjoaisdj”.match(/.\*?sdj/)[0] =\> “asidjoaisdj” Therefore, it should work as Wes expects. -Justin ---
**Author:** ![weyus](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7ab992/32.png) [@weyus](https://www.ruby-forum.com/u/weyus)\ **Post date:** [August 4, 2006, 5:06pm UTC](https://www.ruby-forum.com/t/non-greediness-in-a-regex-need-some-help-verifying-syntax/101276/6 "2006-08-04T17:06:56Z")
Daniel, Currently, I have this working using the .\*? to match everything since I am just passing the results into a block that then does a gsub on the offending characters. Slightly inefficient, but as you pointed out, much more readable. Thanks for the through regex analysis though. Wes ---
**Author:** ![weyus](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7ab992/32.png) [@weyus](https://www.ruby-forum.com/u/weyus)\ **Post date:** [August 4, 2006, 4:30pm UTC](https://www.ruby-forum.com/t/non-greediness-in-a-regex-need-some-help-verifying-syntax/101276/7 "2006-08-04T16:30:28Z")
Wes G. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes: > If I do /~^LNK:._?[\t\r\n]+?._?^~/m - that should pick up what I want, > correct? Almost. The problem is that with this text: a = “~^LNK:foo^~\n\n~^LNK:bar^~” You get a match of the whole text: irb(main):009:0\> a.scan(/~^LNK:._?[\t\r\n]+?._?^~/m) =\> [“~^LNK:foo^~\n\n~^LNK:bar^~”] Where you obviously wanted to get no matches. So, here’s what I suggest: /~^LNK:(?:[^\t\r\n^]|^(?!~))_[\t\r\n]._?^~/m Read that as: ‘~^LNK:’ followed by zero or more of: Some character that isn’t \t, \r, \n, or ‘^’, OR A ‘^’ character that isn’t followed by a ‘~’ Then a \t, \r, or \n character. Then whatever is the minimum other characters necessary to get to ^~. For these “containing at least one of” type problems, I often find it useful to write the regular expression as: begin sequence ( ~^LNK: ) zero or more characters with none of what we want ( (?:[^\t\r\n^]|^(?!~))\* ) one of what we want ( [\t\r\n] ) ._? ( ._? ) end sequence ( ^~ ) For the related “at least n of” problem, (where n \> 1), I do this: begin sequence (?: zero or more characters with none of what we want one of what we want ){n} .\*? end sequence The only tricky part is inside the “none of what we want” chunk, where you have to take care that the “none of what we want” chunk can’t swallow up your end sequence. (Depending on what you want and what your end sequence is, you also need to be careful that the “one of what we want” part can’t swallow part of your end sequence) Sometimes it’s easier to just write a regular expression that gets more matches than you want, and then throw away excess matches in code: lnk\_regex = /~^LNK:.\*?^~/ text.scan(lnk\_regex) { |m| next unless m[0] =~ /[\t\r\n]/ … } That can often be more readable too. Depending on your data, however, it may be much, much slower than using a regular expression that finds only what you need to begin with.