Grunt task automation for multiple environments

Here is my grunt file which is not showing error but not working properly, if i remove dev and prod from 'string-replace' and have only one 'string-replace', then its work.Suggest any solution for multiple environmet.

//Gruntfile.js

module.exports = function(grunt) {
    grunt.initConfig({
        pkg: grunt.file.readJSON('package.json'),

    'string-replace': {
        dev : {
            inline: {
                files: {
                    'index.html':'index.html'
                },
                options: {
                    replacements: [
                        {
                            pattern: '<!--start PROD imports-->',
                            replacement: '<!--start PROD imports'
                        },
                        {
                            pattern: '<!--end PROD imports-->',
                            replacement: 'end PROD imports-->'
                        },
                        {
                            pattern: '<!--start DEV imports',
                            replacement: '<!--start DEV imports-->'
                        },
                        {
                            pattern: 'end DEV imports-->',
                            replacement: '<!--end DEV imports-->'
                        }
                        ]
                }
            }
        },
        prod : {
            inline: {
                files: {
                    'index.html':'index.html'
                },
                options: {
                    replacements: [
                        {
                            pattern: '<!--start PROD imports',
                            replacement: '<!--start PROD imports-->'
                        },
                        {
                            pattern: 'end PROD imports-->',
                            replacement: '<!--end PROD imports-->'
                        },
                        {
                            pattern: '<!--start DEV imports-->',
                            replacement: '<!--start DEV imports'
                        },
                        {
                            pattern: '<!--end DEV imports-->',
                            replacement: 'end DEV imports-->'
                        }
                        ]
                }
            }
        }
    }
});
    grunt.loadNpmTasks('grunt-string-replace');

    grunt.registerTask('default', ['string-replace:dev']);
    grunt.registerTask('prod', ['string-replace:prod']);
};

I need to have it for dev and production both.

via Shubham Tripathi