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Java's Regex.Pattern supports the following character class:
[a-z&&[def]]
which matches "d, e, or f" and is called an intersection.
Functionally this is no different from:
[def]
which is simpler to read and understand in a big RE. So my question is, what use are intersections, other than specifying complete support for CSG-like operations on character classes?
(Please note, I understand the utility of subtractions like [a-z&&[^bc]] and [a-z&&[^m-p]], I am asking specifically about intersections as presented above.)
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In the context of regular expressions, a character class is a set of characters enclosed within square brackets. It specifies the characters that will successfully match a single character from a given input string.
Backslashes in Java. The backslash \ is an escape character in Java Strings. That means backslash has a predefined meaning in Java. You have to use double backslash \\ to define a single backslash. If you want to define \w , then you must be using \\w in your regex.
Character classes match any symbol from certain character sets e.g., \d , \s , and \w . The character classes \d , \s , and \w have the inverse classes \D , \S and \W that match other characters except \d , \s and \w . The dot( . ) matches any character except the newline character.
\\. matches the literal character . . the first backslash is interpreted as an escape character by the Emacs string reader, which combined with the second backslash, inserts a literal backslash character into the string being read. the regular expression engine receives the string \. html?\ ' .
Though I've never had the need to do so, I could imagine a use with pre-defined character classes that aren't proper subsets of each other (thus making the intersection produce something different than the original two character classes). E.g. matching only lower case Latin characters:
[\p{Ll}&&\p{InBasicLatin}]
I believe that particular sample is just a "proof of concept." Two intersected character classes only match a character that matches both character sets individually. The substractions you mentioned are the real practical applications of the operator.
Simply put, there is no hidden meaning.
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