To check if a string is alphanumeric in javascript

Try this regex:

/((^[0-9]+[a-z]+)|(^[a-z]+[0-9]+))+[0-9a-z]+$/i

Which allows only Alphanumeric.

It doesn't allow:

• Only Alpha
• Only Numbers

Refer LIVE DEMO

Updated:

Below regex allows:

/^([0-9]|[a-z])+([0-9a-z]+)$/i

• AlphaNumeric
• Only Alpha
• Only Numbers

Felix Kling's answer is the best single-regex solution in my opinion, but I would consider doing it with three patterns. This allows you to return useful error messages:

if(pwd.match(/[^0-9a-z]/i))
    alert("Only letters and digits allowed!");
else if(!pwd.match(/\d/))
    alert("At least one digit required!");
else if(!pwd.match(/[a-z]/i))
    alert("At least one letter required!");
else
    // process pwd

You should consider allowing non-alphanumeric characters for your passwords though - that would make them a lot safer.

You can use lookaheads:

/^(?=.*?[a-z])(?=.*?\d)[a-z\d]+$/i
   ^^^^^^^^^^  ^^^^^^^
    at least   at least
   one letter  one digit

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FYI, restricting the allowed characters for a password reduce the entropy a lot (there are only 36 different characters now) and hence makes them much easier to crack. Don't do this restriction. Checking whether the string contains a certain type of character is fine though (well, there are some theories that this reduces entropy as well, but I don't have enough knowledge about that).

See also: Regex to accept atleast one alphabet one numeric char and one special Character