Add "Optimization" section to regexp.rdoc (ruby#8849)

* Add "Optimization" section to regexp.rdoc

* Apply the suggestions by @BurdetteLamar

---------

Co-authored-by: Burdette Lamar <[email protected]>

doc/regexp.rdoc

@@ -1228,6 +1228,33 @@ when regexp.timeout is non-+nil+, that value controls timing out:
   |         nil          |         Float        | Times out in Float seconds. |
   |        Float         |          Any         | Times out in Float seconds. |
 
+== Optimization
+
+For certain values of the pattern and target string,
+matching time can grow polynomially or exponentially in relation to the input size;
+the potential vulnerability arising from this is the {regular expression denial-of-service}[https://en.wikipedia.org/wiki/ReDoS] (ReDoS) attack.
+
+\Regexp matching can apply an optimization to prevent ReDoS attacks.
+When the optimization is applied, matching time increases linearly (not polynomially or exponentially)
+in relation to the input size, and a ReDoS attach is not possible.
+
+This optimization is applied if the pattern meets these criteria:
+
+- No backreferences.
+- No subexpression calls.
+- No nested lookaround anchors or atomic groups.
+- No nested quantifiers with counting (i.e. no nested <tt>{n}</tt>,
+  <tt>{min,}</tt>, <tt>{,max}</tt>, or <tt>{min,max}</tt> style quantifiers)
+
+You can use method Regexp.linear_time? to determine whether a pattern meets these criteria:
+
+  Regexp.linear_time?(/a*/)     # => true
+  Regexp.linear_time?('a*')     # => true
+  Regexp.linear_time?(/(a*)\1/) # => false
+
+However, an untrusted source may not be safe even if the method returns +true+,
+because the optimization uses memoization (which may invoke large memory consumption).
+
 == References
 
 Read (online PDF books):