使用 gsub 和块替换模式

Question

When you have strings like this:

str = "john_1"
str = "joe_34"
str = "jack_54"

And you want to get all characters preceding the underscore, you can use split:

str.split("_")[0]
 => "john" 

You can also achieve the same using matched groups and \\1 \\2 matchers:

str = "john_1"
str.gsub(/(.+)_.+/,'\1')

I am trying to understand what happens in the block better. From what I read, it replaces the matched group with the result of executing the block. But I don't see it behaving that way.

In the below example, if it is supposed to replace the matched group with the result of the block:

str = "john_1"
str.gsub(/.+(_.+)/) {|m| ""}
 => "" 

Then why is the return value an empty string? The result should be "john" because only the underscore and all subsequent characters were matched and replaced with an empty string. What am I missing?

Answer 1

gsub replaces all occurrences of input pattern with the value of the result of the block. Here /.+(_.+)/ will match the whole string john_1 , hence the whole string will be replaced with ""

If you want to only match _1 part with regex, here is one way to do it with regex lookahead

str.gsub(/(?=.+)(_.+)/, "")