Add Extract Capture Group Matches With String Slices as a Ruby TIL

ruby/extract-capture-group-matches-with-string-slices.md

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+# Extract Capture Group Matches With String Slices
+
+Ruby's _string slice_ syntax allows us to use the square brackets to access
+portions of a string. It's most common to pass positional integer index
+arguments or a range. However, in true Ruby fashion, another way of thinking
+about defining the slice of a string is based on a regex match.
+
+We can pass a regex and an int (specifying which match we want) to extract some
+portion of a string based on the regex match. That includes capture groups.
+
+Here are a couple examples of extracting matching capture groups as well as
+getting the entire regex match:
+
+```ruby
+> "me+abc123@email.com"[/.+\+(.+)@(.+)/, 1]
+=> "abc123"
+
+> "me+abc123@email.com"[/.+\+(.+)@(.+)/, 2]
+=> "email.com"
+
+> "me+abc123@email.com"[/.+\+(.+)@(.+)/, 0]
+=> "me+abc123@email.com"
+
+> "me+abc123@email.com"[/.+\+(.+)@(.+)/]
+=> "me+abc123@email.com"
+```
+
+The `0`th match (which is the default) corresponds to the full match. Each
+integer position after that corresponds to any capture groups. This maps
+directly to the underlying `MatchData` object:
+
+```ruby
+> /.+\+(.+)@(.+)/.match("me+abc123@email.com")
+=> #<MatchData "me+abc123@email.com" 1:"abc123" 2:"email.com">
+```
+
+[source](https://ruby-doc.org/3.3.6/String.html#class-String-label-String+Slices)