First lets see the code
Now the explanation
/^([a-zA-Z0-9_\.\-])+\@+([a-zA-Z0-9\-]{2,})+\.+([a-zA-Z0-9\.]{2,6})$/
| ^[a-zA-Z0-9_\.\-] | = | Matching the beginning of the string Any character from a-z, A-Z, 0-9 and . – _ |
| \@ | = | @ |
| [a-zA-Z0-9\-]{2,} | = | Any character from a-z, A-Z, 0-9 and – Number of characters should be at least 2 There are no upper limit on number of characters |
| \. | = | . |
| ([a-zA-Z0-9\.]{2,6})$ | = | Matching the end of the string Any character from a-z, A-Z, 0-9 and . Number of characters should be between 2 and 6 |