Split and RegExp
Split and RegExp and Delimiters
Use Java’s split method is easy. You split a string at the delimiters.
"a/b/c".split("/")
does what you expect it to do, an array containing [a, b, c].
But what if you want to return the delimiters as well. Now we have to use some more regular expression skills.
return delimiters separately
For this staring string we could use \b as the regular expression. This matches word boundaries. So you are able to split our original string between the characters and the /.
"a/b/c".split("\\b")
returns [a, /, b, /, c].
Note: Using regular expressions, you are able to select not only indexes of your original string but between indexes as well. In the above example the word boundary is between a and / or / and b.
Interludium: lookahead and lookbehind of regular expressions
Using lookahead or lookbehind you requesting the context of your match. But the result will not be part of the match itself. We call those non-capturing. Both types are available in positive or negative form, which means looking for something that is there or is not there. Look into the following table to get a grip on that.
| name | expression | explanation |
|---|---|---|
| positive lookahead | (?=X) | This expression is useful, if you want to match something which is followed by something else, in this case a X. So to say you are finding all things before a X. |
| negative lookahead | (?!X) | This one finds all things that are not followed by a X. |
| positive lookbehind | (?<=X) | This one finds all things that are following a X. |
| negative lookbehind | (?<!X) | This one finds all things that not are following a X. |
split before a /
Using our lookahead / lookbehind knowledge we are able to build that kind of regular expression. So we want to split before a /, that means at a position, that is followed by a /. Therefore we use a lookahead.
"a/b/c".split("(?=/)")
returns [a, /b, /c].
split after a /
Now we want to split at positions that follow a /. Remember, that this position is somehow between the charaters indexes. Here we need a lookbehind.
"a/b/c".split("(?<=/)")
returns [a/, b/, c].
Conclusion
Using special regular expression, we are able to return delimiters of your strings as well in various forms. We love regular expressions ;).