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Can someone please explain the meaning of \\A and \\z to me? I am assuming that they have a special meaning in this regular expression because they are being escaped (but I could be wrong and they could just stand for A and z, respectively). Thanks!
private static final Pattern PATTERN = Pattern.compile("\\A(\\d+)\\.(\\d+)\\z");
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A regular expression is a sequence of characters that forms a search pattern. When you search for data in a text, you can use this search pattern to describe what you are searching for. A regular expression can be a single character, or a more complicated pattern.
$ means "Match the end of the string" (the position after the last character in the string).
The plus sign + is a greedy quantifier, which means one or more times. For example, expression X+ matches one or more X characters. Therefore, the regular expression \s matches a single whitespace character, while \s+ will match one or more whitespace characters.
For example, \d means a range of digits (0-9), and \w means a word character (any lowercase letter, any uppercase letter, the underscore character, or any digit).
\A means "start of string", and \z means "end of string".
You might have seen ^ and $ in this context, but their meaning can vary: If you compile a regex using Pattern.MULTILINE, then they change their meaning to "start of line" and "end of line". The meaning of \A and \z never changes.
There also exists \Z which means "end of string, before any trailing newlines", which is similar to what $ does in multiline mode (where it matches right before the line-ending newline character, if it's there).
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