diff --git a/README.md b/README.md
index de43c85..2326dd6 100644
--- a/README.md
+++ b/README.md
@@ -21,19 +21,33 @@ var app = express();
 
 app.use(bodyParser.urlencoded({extended: true}));
 app.use(bodyParser.json());
+
+// To remove data, use:
 app.use(mongoSanitize());
 
+// Or, to replace prohibited characters with _, use:
+app.use(mongoSanitize({
+  replaceWith: '_'
+}))
+
 ```
 
 ## What?
 
-This module removes any keys in objects that begin with a `$` sign from `req.body`, `req.query` or `req.params`.
+This module searches for any keys in objects that begin with a `$` sign or contain a `.`, from `req.body`, `req.query` or `req.params`. It can then either:
+
+- completely remove these keys and associated data from the object, or
+- replace the prohibited characters with another allowed character.
+
+The behaviour is governed by the passed option, `replaceWith`. Set this option to have the sanitizer replace the prohibited characters with the character passed in.
+
+See the spec file for more examples.
 
 ## Why?
 
-Object keys starting with a `$` are _reserved_ for use by MongoDB as operators. Without this sanitization,  malicious users could send an object containing a `$` operator, which could change the context of a database operation. Most notorious is the `$where` operator, which can execute arbitrary JavaScript on the database.
+Object keys starting with a `$` or containing a `.` are _reserved_ for use by MongoDB as operators. Without this sanitization,  malicious users could send an object containing a `$` operator, or including a `.`, which could change the context of a database operation. Most notorious is the `$where` operator, which can execute arbitrary JavaScript on the database.
 
-The best way to prevent this is to sanitize the received data, and remove any offending keys.
+The best way to prevent this is to sanitize the received data, and remove any offending keys, or replace the characters with a 'safe' one.
 
 ## Credits
 
diff --git a/index.js b/index.js
index aafae51..0eab9fd 100644
--- a/index.js
+++ b/index.js
@@ -1,29 +1,52 @@
 'use strict';
 
-var sanitize = function(val) {
-  if(Array.isArray(val)) {
-    val.forEach(sanitize);
-
-  } else if(val instanceof Object) {
-    Object.keys(val).forEach(function(key) {
-      if (/^\$|\./.test(key)) {
-        delete val[key];
-      } else {
-        sanitize(val[key]);
-      }
-    });
+var TEST_REGEX = /^\$|\./,
+    REPLACE_REGEX = /^\$|\./g;
+
+var sanitize = function(val, options) {
+  options = options || {};
+
+  var replaceWith = null;
+  if(!(TEST_REGEX.test(options.replaceWith))) {
+    replaceWith = options.replaceWith;
   }
 
-  return val;
+  var act = function(val) {
+    if(Array.isArray(val)) {
+      val.forEach(act);
+
+    } else if(val instanceof Object) {
+      Object.keys(val).forEach(function(key) {
+        var v = val[key];
+        var noRecurse = false;
+
+        if(TEST_REGEX.test(key)) {
+          delete val[key];
+          if(replaceWith) {
+            val[key.replace(REPLACE_REGEX, replaceWith)] = v;
+          } else {
+            noRecurse = true;
+          }
+        }
+
+        if(!noRecurse) {
+          act(v);
+        }
+
+      });
+    }
+
+    return val;
+  };
+
+  return act(val);
 };
 
 var middleware = function(options) {
-  options = options || {};
-
   return function(req, res, next) {
     ['body', 'params', 'query'].forEach(function(k) {
       if(req[k]) {
-        req[k] = sanitize(req[k]);
+        req[k] = sanitize(req[k], options);
       }
     });
     next();
diff --git a/test.js b/test.js
index c431d4e..7235038 100644
--- a/test.js
+++ b/test.js
@@ -6,156 +6,387 @@ var request = require('supertest'),
     sanitize = require('./index.js');
 
 describe('Express Mongo Sanitize', function() {
-  var app = express();
-  app.use(bodyParser.urlencoded({extended: true}));
-  app.use(bodyParser.json());
-  app.use(sanitize());
-
-  app.post('/body', function(req, res){
-    res.status(200).json({
-      body: req.body
-    });
-  });
+  describe('Remove Data', function() {
+    var app = express();
+    app.use(bodyParser.urlencoded({extended: true}));
+    app.use(bodyParser.json());
+    app.use(sanitize());
 
-  app.get('/query', function(req, res){
-    res.status(200).json({
-      query: req.query
+    app.post('/body', function(req, res){
+      res.status(200).json({
+        body: req.body
+      });
     });
-  });
 
-  describe('Top-level object', function() {
-    it('should sanitize the query string', function(done) {
-      request(app)
-        .get('/query?q=search&$where=malicious&dotted.data=some_data')
-        .set('Accept', 'application/json')
-        .expect(200, {
-          query: {
-            q: 'search'
-          }
-        }, done);
+    app.get('/query', function(req, res){
+      res.status(200).json({
+        query: req.query
+      });
     });
 
-    it('should sanitize a JSON body', function(done) {
-      request(app)
-        .post('/body')
-        .send({
-          q: 'search',
-          is: true,
-          and: 1,
-          even: null,
-          stop: undefined,
-          $where: 'malicious',
-          'dotted.data': 'some_data'
-        })
-        .set('Content-Type', 'application/json')
-        .set('Accept', 'application/json')
-        .expect(200, {
-          body: {
+    describe('Top-level object', function() {
+      it('should sanitize the query string', function(done) {
+        request(app)
+          .get('/query?q=search&$where=malicious&dotted.data=some_data')
+          .set('Accept', 'application/json')
+          .expect(200, {
+            query: {
+              q: 'search'
+            }
+          }, done);
+      });
+
+      it('should sanitize a JSON body', function(done) {
+        request(app)
+          .post('/body')
+          .send({
             q: 'search',
             is: true,
             and: 1,
-            even: null
-          }
-        }, done);
+            even: null,
+            stop: undefined,
+            $where: 'malicious',
+            'dotted.data': 'some_data'
+          })
+          .set('Content-Type', 'application/json')
+          .set('Accept', 'application/json')
+          .expect(200, {
+            body: {
+              q: 'search',
+              is: true,
+              and: 1,
+              even: null
+            }
+          }, done);
+      });
+
+      it('should sanitize a form url-encoded body', function(done) {
+        request(app)
+          .post('/body')
+          .send('q=search&$where=malicious&dotted.data=some_data')
+          .set('Content-Type', 'application/x-www-form-urlencoded')
+          .set('Accept', 'application/json')
+          .expect(200, {
+            body: {
+              q: 'search'
+            }
+          }, done);
+      });
     });
 
-    it('should sanitize a form url-encoded body', function(done) {
-      request(app)
-        .post('/body')
-        .send('q=search&$where=malicious&dotted.data=some_data')
-        .set('Content-Type', 'application/x-www-form-urlencoded')
-        .set('Accept', 'application/json')
-        .expect(200, {
-          body: {
-            q: 'search'
-          }
-        }, done);
+    describe('Nested Object', function() {
+      it('should sanitize a nested object in the query string', function(done) {
+        request(app)
+          .get('/query?username[$gt]=foo&username[dotted.data]=some_data')
+          .set('Accept', 'application/json')
+          .expect(200, {
+            query: {
+              username: {}
+            }
+          }, done);
+      });
+
+      it('should sanitize a nested object in a JSON body', function(done) {
+        request(app)
+          .post('/body')
+          .send({
+            username: {
+              $gt: 'foo',
+              'dotted.data': 'some_data'
+            }
+          })
+          .set('Content-Type', 'application/json')
+          .set('Accept', 'application/json')
+          .expect(200, {
+            body: {
+              username: {}
+            }
+          }, done);
+      });
+
+      it('should sanitize a nested object in a form url-encoded body', function(done) {
+        request(app)
+          .post('/body')
+          .send('username[$gt]=foo&username[dotted.data]=some_data')
+          .set('Content-Type', 'application/x-www-form-urlencoded')
+          .set('Accept', 'application/json')
+          .expect(200, {
+            body: {
+              username: {}
+            }
+          }, done);
+      });
+    });
+
+    describe('Nested Object inside an Array', function() {
+      it('should sanitize a nested object in the query string', function(done) {
+        request(app)
+          .get('/query?username[0][$gt]=foo&username[0][dotted.data]=some_data')
+          .set('Accept', 'application/json')
+          .expect(200, {
+            query: {
+              username: [{}]
+            }
+          }, done);
+      });
+
+      it('should sanitize a nested object in a JSON body', function(done) {
+        request(app)
+          .post('/body')
+          .send({
+            username: [{
+              $gt: 'foo',
+              'dotted.data': 'some_data'
+            }]
+          })
+          .set('Content-Type', 'application/json')
+          .set('Accept', 'application/json')
+          .expect(200, {
+            body: {
+              username: [{}]
+            }
+          }, done);
+      });
+
+      it('should sanitize a nested object in a form url-encoded body', function(done) {
+        request(app)
+          .post('/body')
+          .send('username[0][$gt]=foo&username[0][dotted.data]=some_data')
+          .set('Content-Type', 'application/x-www-form-urlencoded')
+          .set('Accept', 'application/json')
+          .expect(200, {
+            body: {
+              username: [{}]
+            }
+          }, done);
+      });
     });
   });
 
-  describe('Nested Object', function() {
-    it('should sanitize a nested object in the query string', function(done) {
-      request(app)
-        .get('/query?username[$gt]=foo&username[dotted.data]=some_data')
-        .set('Accept', 'application/json')
-        .expect(200, {
-          query: {
-            username: {}
-          }
-        }, done);
+  describe('Preserve Data', function() {
+    var app = express();
+    app.use(bodyParser.urlencoded({extended: true}));
+    app.use(bodyParser.json());
+    app.use(sanitize({
+      replaceWith: '_'
+    }));
+
+    app.post('/body', function(req, res){
+      res.status(200).json({
+        body: req.body
+      });
     });
 
-    it('should sanitize a nested object in a JSON body', function(done) {
-      request(app)
-        .post('/body')
-        .send({
-          username: {
-            $gt: 'foo',
+    app.get('/query', function(req, res){
+      res.status(200).json({
+        query: req.query
+      });
+    });
+
+    describe('Top-level object', function() {
+      it('should sanitize the query string', function(done) {
+        request(app)
+          .get('/query?q=search&$where=malicious&dotted.data=some_data')
+          .set('Accept', 'application/json')
+          .expect(200, {
+            query: {
+              q: 'search',
+              _where: 'malicious',
+              dotted_data: 'some_data'
+            }
+          }, done);
+      });
+
+      it('should sanitize a JSON body', function(done) {
+        request(app)
+          .post('/body')
+          .send({
+            q: 'search',
+            is: true,
+            and: 1,
+            even: null,
+            stop: undefined,
+            $where: 'malicious',
             'dotted.data': 'some_data'
-          }
-        })
-        .set('Content-Type', 'application/json')
-        .set('Accept', 'application/json')
-        .expect(200, {
-          body: {
-            username: {}
-          }
-        }, done);
+          })
+          .set('Content-Type', 'application/json')
+          .set('Accept', 'application/json')
+          .expect(200, {
+            body: {
+              q: 'search',
+              is: true,
+              and: 1,
+              even: null,
+              _where: 'malicious',
+              dotted_data: 'some_data'
+            }
+          }, done);
+      });
+
+      it('should sanitize a form url-encoded body', function(done) {
+        request(app)
+          .post('/body')
+          .send('q=search&$where=malicious&dotted.data=some_data')
+          .set('Content-Type', 'application/x-www-form-urlencoded')
+          .set('Accept', 'application/json')
+          .expect(200, {
+            body: {
+              q: 'search',
+              _where: 'malicious',
+              dotted_data: 'some_data'
+            }
+          }, done);
+      });
     });
 
-    it('should sanitize a nested object in a form url-encoded body', function(done) {
-      request(app)
-        .post('/body')
-        .send('username[$gt]=foo&username[dotted.data]=some_data')
-        .set('Content-Type', 'application/x-www-form-urlencoded')
-        .set('Accept', 'application/json')
-        .expect(200, {
-          body: {
-            username: {}
-          }
-        }, done);
+    describe('Nested Object', function() {
+      it('should sanitize a nested object in the query string', function(done) {
+        request(app)
+          .get('/query?username[$gt]=foo&username[dotted.data]=some_data')
+          .set('Accept', 'application/json')
+          .expect(200, {
+            query: {
+              username: {
+                _gt: 'foo',
+                dotted_data: 'some_data'
+              }
+            }
+          }, done);
+      });
+
+      it('should sanitize a nested object in a JSON body', function(done) {
+        request(app)
+          .post('/body')
+          .send({
+            username: {
+              $gt: 'foo',
+              'dotted.data': 'some_data'
+            }
+          })
+          .set('Content-Type', 'application/json')
+          .set('Accept', 'application/json')
+          .expect(200, {
+            body: {
+              username: {
+                _gt: 'foo',
+                dotted_data: 'some_data'
+              }
+            }
+          }, done);
+      });
+
+      it('should sanitize a nested object in a form url-encoded body', function(done) {
+        request(app)
+          .post('/body')
+          .send('username[$gt]=foo&username[dotted.data]=some_data')
+          .set('Content-Type', 'application/x-www-form-urlencoded')
+          .set('Accept', 'application/json')
+          .expect(200, {
+            body: {
+              username: {
+                _gt: 'foo',
+                dotted_data: 'some_data'
+              }
+            }
+          }, done);
+      });
+    });
+
+    describe('Nested Object inside an Array', function() {
+      it('should sanitize a nested object in the query string', function(done) {
+        request(app)
+          .get('/query?username[0][$gt]=foo&username[0][dotted.data]=some_data')
+          .set('Accept', 'application/json')
+          .expect(200, {
+            query: {
+              username: [{
+                _gt: 'foo',
+                dotted_data: 'some_data'
+              }]
+            }
+          }, done);
+      });
+
+      it('should sanitize a nested object in a JSON body', function(done) {
+        request(app)
+          .post('/body')
+          .send({
+            username: [{
+              $gt: 'foo',
+              'dotted.data': 'some_data'
+            }]
+          })
+          .set('Content-Type', 'application/json')
+          .set('Accept', 'application/json')
+          .expect(200, {
+            body: {
+              username: [{
+                _gt: 'foo',
+                dotted_data: 'some_data'
+              }]
+            }
+          }, done);
+      });
+
+      it('should sanitize a nested object in a form url-encoded body', function(done) {
+        request(app)
+          .post('/body')
+          .send('username[0][$gt]=foo&username[0][dotted.data]=some_data')
+          .set('Content-Type', 'application/x-www-form-urlencoded')
+          .set('Accept', 'application/json')
+          .expect(200, {
+            body: {
+              username: [{
+                _gt: 'foo',
+                dotted_data: 'some_data'
+              }]
+            }
+          }, done);
+      });
     });
   });
 
-  describe('Nested Object inside an Array', function() {
-    it('should sanitize a nested object in the query string', function(done) {
-      request(app)
-        .get('/query?username[0][$gt]=foo&username[0][dotted.data]=some_data')
+  describe('Preserve Data: prohibited characters', function() {
+    it('should not allow data to be replaced with a `$`', function(done) {
+      var app = express();
+      app.use(bodyParser.urlencoded({extended: true}));
+      app.use(sanitize({
+        replaceWith: '$'
+      }));
+
+      app.get('/query', function(req, res){
+        res.status(200).json({
+          query: req.query
+        });
+      });
+       request(app)
+        .get('/query?q=search&$where=malicious&dotted.data=some_data')
         .set('Accept', 'application/json')
         .expect(200, {
           query: {
-            username: [{}]
+            q: 'search'
           }
         }, done);
     });
 
-    it('should sanitize a nested object in a JSON body', function(done) {
-      request(app)
-        .post('/body')
-        .send({
-          username: [{
-            $gt: 'foo',
-            'dotted.data': 'some_data'
-          }]
-        })
-        .set('Content-Type', 'application/json')
-        .set('Accept', 'application/json')
-        .expect(200, {
-          body: {
-            username: [{}]
-          }
-        }, done);
-    });
+    it('should not allow data to be replaced with a `.`', function(done) {
+      var app = express();
+      app.use(bodyParser.urlencoded({extended: true}));
+      app.use(sanitize({
+        replaceWith: '.'
+      }));
 
-    it('should sanitize a nested object in a form url-encoded body', function(done) {
-      request(app)
-        .post('/body')
-        .send('username[0][$gt]=foo&username[0][dotted.data]=some_data')
-        .set('Content-Type', 'application/x-www-form-urlencoded')
+      app.get('/query', function(req, res){
+        res.status(200).json({
+          query: req.query
+        });
+      });
+       request(app)
+        .get('/query?q=search&$where=malicious&dotted.data=some_data')
         .set('Accept', 'application/json')
         .expect(200, {
-          body: {
-            username: [{}]
+          query: {
+            q: 'search'
           }
         }, done);
     });