正则表达式中下划线字符背后的含义

Question

Note: My program works, I am just looking for an explanation as to why.

I have a java program that reads a file, counts the words in said file, and outputs the words and counts to another file. In the first part of my I use a regular expression to replace any character not a letter and replace it with an empty string.

freq.add(in.next().replaceAll("[^A-Za-z]", ""));

This however does not account for hyphenated words so I changed the regEx to:

freq.add(in.next().replaceAll("[^A-Za-z_-]", ""));

My question is, why does adding the underscore and hyphen work? What is the meaning behind the underscore character?

While I'm asking questions, are regex the same for all languages?

Also, if this is answered somewhere else I apologize, I did numerous searches with no luck.

Answer 1

There's nothing special about an underscore in a regular expression, it's just a normal character like A. A hyphen at the end of a character class isn't special either, although it is when between two other characters, as you've used it to match all letters by saying AZ for example.

Regular expressions are similar between most languages, but some of the more esoteric features can be different or missing from a language.