allow only alphabets and underscore using javascript

I am开发者_C百科 allowing a string to contain only alphabets and underscore,but is i enter fist character as alphabet or underscore and later if i put any invalid character then this validation is being done.I have done validation as follows:

function permission_validate()
{var permission=document.permissionForm.permission.value;var allowedStr=/[A-Za-z_]/;

if(!allowedStr.test(permission)){document.getElementById("permission_Er").innerHTML="* Required field can contain Only A-Z/az/_";

document.permissionForm.permission.focus();return false;}else{return true;}

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The correct regex to use would be

/^[a-zA-Z\_]+$/g

^ matches beginning
$ matches end
g matches the whole string.

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/^[a-z_]+$/gi

Note: You need anchors ^ and $ and + to accept one of more characters of alphabets and underscores.