console.log( 'test -test- !test!'.replace(/\btest\b/g, '1234') );
// 1234 -1234- !1234!

console.log( '0test5 atestb _test_ test'.replace(/\btest\b/g, '1234') );
// 0test5 atestb _test_ 1234

console.log( 'test -test- !test!'.replace(/\Btest\B/g, '1234') );
// test -test- !test!

console.log( '0test5 atestb _test_ test'.replace(/\Btest\B/g, '1234') );
// 012345 a1234b _1234_ test