make regexp allowing nil value

README.md

@@ -820,7 +820,7 @@ end
 #### `regexp`
 
 Parameters can be restricted to match a specific regular expression with the `:regexp` option. If the value
-is nil or does not match the regular expression an error will be returned. Note that this is true for both `requires`
+does not match the regular expression an error will be returned. Note that this is true for both `requires`
 and `optional` parameters.
 
 ```ruby
@@ -829,6 +829,14 @@ params do
 end
 ```
 
+`regexp` will not fail if the parameter was sent without value. To ensure that the parameter contain
+a value, use `allow_blank` validator.
+
+```ruby
+params do
+  requires :email, allow_blank: false, regexp: /.+@.+/
+end
+```
 
 #### `mutually_exclusive`
 

lib/grape/validations/validators/regexp.rb

@@ -3,7 +3,7 @@ module Validations
     class RegexpValidator < Base
       def validate_param!(attr_name, params)
         if params.key?(attr_name) &&
-           (params[attr_name].nil? || !(params[attr_name].to_s =~ @option))
+           !params[attr_name].nil? && !(params[attr_name].to_s =~ @option)
           fail Grape::Exceptions::Validation, params: [@scope.full_name(attr_name)], message_key: :regexp
         end
       end

spec/grape/validations/validators/presence_spec.rb

@@ -55,7 +55,7 @@ def app
   context 'with a required non-empty string' do
     before do
       subject.params do
-        requires :email, type: String, regexp: /^\S+$/
+        requires :email, type: String, allow_blank: false, regexp: /^\S+$/
       end
       subject.get do
         'Hello'
@@ -64,12 +64,12 @@ def app
     it 'requires when missing' do
       get '/'
       expect(last_response.status).to eq(400)
-      expect(last_response.body).to eq('{"error":"email is missing, email is invalid"}')
+      expect(last_response.body).to eq('{"error":"email is missing, email is empty"}')
     end
     it 'requires when empty' do
       get '/', email: ''
       expect(last_response.status).to eq(400)
-      expect(last_response.body).to eq('{"error":"email is invalid"}')
+      expect(last_response.body).to eq('{"error":"email is empty, email is invalid"}')
     end
     it 'valid when set' do
       get '/', email: '[email protected]'

spec/grape/validations/validators/regexp_spec.rb

@@ -25,12 +25,17 @@ def app
       expect(last_response.status).to eq(400)
     end
 
-    it 'refuses nil' do
-      get '/', name: nil
+    it 'refuses empty' do
+      get '/', name: ''
       expect(last_response.status).to eq(400)
     end
   end
 
+  it 'accepts nil' do
+    get '/', name: nil
+    expect(last_response.status).to eq(200)
+  end
+
   it 'accepts valid input' do
     get '/', name: 'bob'
     expect(last_response.status).to eq(200)