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Java Forum / General / January 2008Regexes: Forcing the LAST Match
I'm not sure what terms to look for here. I want to use a regex that will match as little as possible in a string from the *end* of the string. For example, if I have "OneTwoThreeTwoOne", I want to know how I can match only "TwoOne" at the end and not "TwoThreeTwoOne". I've been experimenting with quantifiers, but something like "Two.*?$" will grab everything from the first occurrence of "Two". How can I make it grab from only the last occurrence? Thanks! Hal >I'm not sure what terms to look for here. I want to use a regex that will >match as little as possible in a string from the *end* of the string. »As little as possible from the end of the string« would be "". >For example, if I have "OneTwoThreeTwoOne", I want to know how >I can match only "TwoOne" at the end and not "TwoThreeTwoOne". When I tell you that this can be done by »TwoOne$«, you will not be satisfied, I guess. But it is the actual answer to the preceding English sentence. It might help to try to express what you want in English, in a manner that does not need examples to be understood. »As little as possible from the end« does not seem to be that expression, because it contradicts the example given. >>I'm not sure what terms to look for here. I want to use a regex that will >>match as little as possible in a string from the *end* of the string. [quoted text clipped - 10 lines] > It might help to try to express what you want in English, > in a manner that does not need examples to be understood. Okay. I want to be able to specify a phrase with a regex and remove from the last occurrence of that phrase to the end of the original string. Examples: Full String: "The date 2008-01-07 is one day before Elvis' birthday on 2008-01-08, which is tomorrow." Match: "[0-9]{2,4}-[0-9]{1,2}-[0-9]{1,2}" Desired Result: "The date 2008-01-07 is one day before Elvis' birthday on " It matches the LAST date that fits the format and removes from the last match on. If I used "[0-9]{2,4}-[0-9]{1,2}-[0-9]{1,2}.*?" it'll match from the first date on. Full String: "One Two Three Two One" Match: "Two" Desired Result: "One Two Three " I've tried using negative lookaheads, but, as best I can guess, in that last example, if I use any kind of quantifier, then the first match I get is "Two Three Two One" and it seems to not see it can match just "Two One". I have used Match.find() and a loop to get the last position of the match, then used Match.start() to get the position, then get a substring of the original string, using that start position as where the substring ends, but if I want to delete from just after the match, then I have to check to make sure I'm not out of bounds and so on. I would think there would be an easy way to match the last occurrence of a phrase to the end of a string instead of having to loop through it. Thanks! Hal >>> I'm not sure what terms to look for here. I want to use a regex that will >>> match as little as possible in a string from the *end* of the string. [quoted text clipped - 40 lines] > way to match the last occurrence of a phrase to the end of a string instead > of having to loop through it. In your case you can use this http://java.sun.com/javase/6/docs/api/java/lang/String.html#lastIndexOf(java.lan g.String) robert >>>> I'm not sure what terms to look for here. I want to use a regex that >>>> will match as little as possible in a string from the *end* of the [quoted text clipped - 46 lines] > > In your case you can use this http://java.sun.com/javase/6/docs/api/java/lang/String.html#lastIndexOf(java.lan g.String) But that uses a String not a regex. I tried it. Thanks, though. Hal >I want to be able to specify a phrase with a regex and remove from the last >occurrence of that phrase to the end of the original string. Thus, for the phrase »alpha« and the text »alpha beta alpha gamma alpha delta alpha epsilon«, the text to be removed is »alpha epsilon«. public class Main { public static void test( final java.lang.String text ) { final java.util.regex.Matcher matcher = java.util.regex.Pattern.compile ( "^(.*)alpha.*$" ). matcher( text ); while( matcher.find() ) java.lang.System.out.println( matcher.group( 1 )); } public static void main( final java.lang.String[] args ) { test ( "alpha beta alpha gamma alpha delta alpha epsilon alpha zeta" ); }} alpha beta alpha gamma alpha delta alpha epsilon >>I want to be able to specify a phrase with a regex and remove from the >>last occurrence of that phrase to the end of the original string. [quoted text clipped - 20 lines] > > alpha beta alpha gamma alpha delta alpha epsilon Then what you're doing, in essence, is using a greedy quantifier at the start to gobble up as much as possible so it only finds the last occurrence and then just using capture to get the text that's to be kept and using that as the replacement. Am I right in how this is working? I see it works, I just want to be sure I understand it clearly. Thanks! Hal >Then what you're doing, in essence, is using a greedy >quantifier at the start to gobble up as much as possible so it >only finds the last occurrence and then just using capture to >get the text that's to be kept and using that as the >replacement. I believe so. >>Then what you're doing, in essence, is using a greedy >>quantifier at the start to gobble up as much as possible so it [quoted text clipped - 3 lines] > > I believe so. Okay. I got it. Thanks! Hal >Okay. I got it. One still might ask, whether there is a way to just inspect the end of the string. I am not absolutely sure, whether the following code really does that, but I would try it this way: public class Main { public static java.lang.String pos( final java.lang.String text ) { final java.util.regex.Matcher matcher = java.util.regex.Pattern.compile ( "(?<=(alpha.{0,2147483642}?)$)" ). matcher( text ); return matcher.find() ? matcher.group( 1 ) : ""; } public static void main( final java.lang.String[] args ) { final java.lang.String source = "alpha beta alpha gamma alpha delta"; final java.lang.String stringToBeRemoved = pos( source ); java.lang.System.out.println( stringToBeRemoved ); final java.lang.String result = source.substring ( 0, source.length() - stringToBeRemoved.length() ); java.lang.System.out.println( result ); }} alpha delta alpha beta alpha gamma I guess you want to look for the difference of greedy vs. reluctant (non-greedy) matches. However, see Stefans reply. 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