regex to lazy match possible empty string

Question

I'd like to get anything between A=[ and first enclosing ]:

> str = "Hello A=[apple] B=[boy] World"
> str.match(/A=\[(.+?)\]/)[1]
 => "apple" 

So far so good.

However, for A=[]:

> str = "Hello A=[] B=[boy] World"
> str.match(/A=\[(.+?)\]/)[1]
 => "] B=[boy" 

How can I get a empty string "", instead of "] B=[boy"?

Answer 1

Use .*? rather than .+?. The former matches zero or more characters, while the latter matches one or more.

"Hello A=[] B=[boy] World".match(/A*=*\[(.*?)\]/)[1] # => ""

Answer 2

I'd use:

str = "Hello A=[apple] B=[boy] World"
str[/A=\[(.*?)\]/, 1] # => "apple"
str = "Hello A=[] B=[boy] World"
str[/A=\[(.*?)\]/, 1] # => ""

I'm not sure why you're using A*=*, because it means "zero or more 'A' followed by zero or more '='" and basically fails to do anything useful, and, in fact, actually opens up a hole to return bad results. See the additional information in the edit below.

Here are variations on the same themes. All are documented in the Regexp and String.[] documentation:

str = "Hello A=[apple] B=[boy] World"
str[/A=\[(.*?)\]/, 1] # => "apple"

/A=\[(.*?)\]/ =~ str 
$1 # => "apple"

str =~ /A=\[(.*?)\]/
$1 # => "apple"

/A=\[(?<in_brackets>.*?)\]/ =~ str
in_brackets # => "apple"

str = "Hello A=[] B=[boy] World"
str[/A=\[(.*?)\]/, 1] # => ""

/A=\[(.*?)\]/ =~ str 
$1 # => ""

/A=\[(?<in_brackets>.*?)\]/ =~ str 
in_brackets # => ""

---

the = was intended to match possible space characters before or after the equal sign, e.g. str = "Hello A= [] B=[boy] World"

Well, *=* isn't how you should do it. You need to tell the regex engine what is optional:

/A *= */

would be correct, as would:

/A ?= ?/

Here's some tests to show you what's happening:

str = "Hello [foo] A = [apple] B=[boy] World"
str[/A*=*\[(.*?)\]/, 1] # => "foo"
str[/A *=*\[(.*?)\]/, 1] # => nil
str[/A *= *\[(.*?)\]/, 1] # => "apple"
str[/A ?= ?\[(.*?)\]/, 1] # => "apple"

Notice that your pattern /A*=*/ allows the engine to match [foo], not A = [apple]. This is because you're telling it to match no A and no = as an option, which opens the hole to grab [foo]. As long as your inputs are exactly two options, with no preceding value inside brackets you'll be safe. If you have an input with that your result will be wrong.

---

str[/A.*=.*\[(.*?)\]/,1]?

Nope:

str = "Hello A=[apple] B=[boy] World"
str[/A.*=.*\[(.*?)\]/,1] # => "boy"

I'd recommend exploring how to use regex via Rubular. Here's a link showing the above example and why it fails.