Backreferences in Java

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Throughout this chapter, you have seen how to use capturing groups to store particular subexpressions and use them in code. Regular expressions also allow you to use the captured groups inside the same expression they are captured with. You use a backreference, as it is called, by referring to the ordinal number of the matching group, like using $1 through $99 in String::replace(), except backreferences use backslashes instead of dollar signs: \1 through \99. The text that matches the captured group is used as the subexpression where the backreference appears instead of the literal characters \1. This snippet demonstrates:
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var re:RegExp = /\b (\w{3}) \b .* \b \1 \b /ix; var testString:String = "far, farther fetched free fun fetched fun free"; trace(testString.match(re)); //fun fetched fun,fun
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The preceding regular expression nds the rst three-letter word that appears at least one more time anywhere in the text. It uses a group to capture a three-letter word, .* , to keep matching through the rest of the string, and nally a backreference to assert that the word is repeated later in the string.
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Part II: Core ActionScript 3.0 Data Types
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Because it uses the word boundary anchor \b, it also ensures that the second time the captured three letters appear, they are in a single word. This disquali es the word far, which appears as part of the word farther. If the expression matches at all, the rst repeated three-letter word is stored in the rst captured group.
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Lookahead and Noncapturing Groups
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These different kinds of groups can be used for interesting purposes.
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Noncapturing Groups
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When you want to use a group for alternation or a quanti er, but the actual contents of the group are not as important to your expression, you can reduce the amount of unnecessarily captured groups by making a group that does not capture: a noncapturing group. This is achieved with the following syntax:
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( :...)
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where your grouped subexpression takes the place of the ellipsis. For example:
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var re:RegExp = /( :i|you|he|she|it|we|they) likes to (\w+)/i; var match:Object = re.exec("We like to party."); trace(match[1]); //party
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This expression nds what it is that someone likes to do. It uses a group to allow for several subjects. However, the rst group is noncapturing, so it does not interfere with the verb, which is captured in the rst position.
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When using a noncapturing group, the contents of the group are still available to backreferences.
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Positive Lookahead Groups
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Beyond noncapturing groups, there are also zero-width groups: groups that try to match the source string without moving ahead in the pattern. ActionScript 3.0 regular expressions have a positive lookahead group, or a zero-width positive lookahead assertion. When you use the syntax
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( =...)
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you ensure that the pattern represented by the ellipsis would match, if you tried to. However, it doesn t perform the match, so that after the lookahead group, you re still talking about the same location in the source text as before it. This is unlike most metasequences you ve seen so far, which match and then continue through the source text. Recall that anchors such as ^ and \b are also zero-width.
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var re:RegExp = /\b ( :\w{3}) \b ( = .* \b \1 \b) \s* (\w+) /x; var testString:String = "far, farther fetched free fun fetched fun free"; trace(testString.match(re)); //fun fetched,fetched
12: Regular Expressions
Here you modi ed a prior snippet to nd the word after the rst three-letter word that repeats. The large lookahead in the middle looks ahead to see if there is, after a sequence of characters, another instance of the three-letter word matched. If this lookahead fails (if there is no duplication of the three-letter word just found), the potential match immediately fails and the next bit of source text is considered. When the lookahead succeeds, as it does with fun, it must have examined the source text through the second instance of the word fun. But when the lookahead group matches positively, the regular expression restores the current position in the source text to the position that was already being considered when the expression encountered the lookahead group: after the word boundary after the rst fun. Then the remainder of the expression is considered. After a lookahead group matches, the pointer in the expression is at the end of the lookahead group in the expression, but the pointer in the source text hasn t moved. It s zero-width. Thus, when the match continues at \s*, the source continues after the rst fun and achieves the goal of getting the word after the rst repeated three- letter word. Incidentally, the snippet also uses a noncapturing group to nd the three-letter word because the goal of the search is no longer the word, but the word immediately following. This way, the only captured group is fetched, the solution to the puzzle. You can also see that the entire match fun fetched does not include text up to the second instance of fun, even though that source text was examined by the lookahead.