why do these two different regex's return different results in Ruby based upon position of underscore

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"]]

I am unsure why the second one with the the underscore at the end behaves differently than the first. How is it being interpreted by RegEx parser to make it different?

Answer 1

It's because you have the hyphen (-) placed in the middle of the character class without being escaped.

Within a character class [], you can place a hyphen (-) as the first or last character. If you place the hyphen anywhere else you need to escape it (\-) in order to 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 don't really need to escape the dot (.) either, so you could rewrite this as..

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

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

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

Answer 2

The hyphen is messing things up, not the underscore.

- is a special character inside a character class, indicating a range. One way to escape it is to put it at the beginning or the end of the class: [...-].

So [_.-] checks for a character, either _ or . or -.

And [.-_] check for a character, in the range "from . to _".

Illustration

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