Why do these two different regular expressions return different results in Ruby based on the underscore position

Question

I have the following :.

[11] pry(main)> "ab BN123-4.56".scan(/BN([0-9_\.-]+)/) => [["123-4.56"]] [12] pry(main)> "ab BN123-4.56".scan(/BN([0-9\.-_]+)/) => [["123"]] 

and not sure why the second with an underline at the end behaves differently than the first. How is this interpreted by the RegEx parser to make it different?

THX

Answer 1

This is because you have a hyphen ( - ) placed in the middle of a character class without escaping.

In the [] character class, you can put a hyphen ( - ) as the first or last character. If you place the hyphen somewhere else, you need to escape it ( \- ) so that it can be matched.

 "ab BN123-4.56".scan(/BN([0-9_\.-]+)/) # => '123-4.56' "ab BN123-4.56".scan(/BN([0-9\.\-_]+)/) # => '123-4.56' 

Note You also do not need to avoid periods ( . ), So you can rewrite this as.

 "ab BN123-4.56".scan(/BN([0-9_.-]+)/) # => '123-4.56' 

Or even the following if you decide to place it in the middle of a character class.

 "ab BN123-4.56".scan(/BN([0-9.\-_]+)/) # => '123-4.56' 

Answer 2

A hyphen is a mess, not an underscore.

- is a special character within a character class indicating a range. One way to avoid this is to put it at the beginning or end of the class: [...-] .

So, [_.-] checks the character, either _ , or . or - .

And [.-_] Check the character in the range from . to _ ".

Illustration

BN([0-9.\-_]+) does what you expect and selects 123-4.56 from ab BN123-4.56 .

Answer 3

A hyphen inside square brackets [] indicates a range. To use a literal hyphen, execute it, as well as special characters with \