Question

I need a regular expression to allow the user to enter an alphanumeric string that starts with a letter (not a digit).

Answer 1

This should work in any of the Regular Expression (RE) engines. There is a nicer syntax in the PCRE world but I prefer mine to be able to run anywhere:

^[A-Za-z][A-Za-z0-9]*$

Basically, the first character must be alpha, followed by zero or more alpha-numerics. The start and end tags are there to ensure that the whole line is matched. Without those, you may match the AB12 of the "@@@AB12!!!" string.

Full explanation:

^            start tag.
[A-Za-z]     any one of the upper/lower case letters.
[A-Za-z0-9]  any one of the upper/lower case letters or digits,
*            repeated zero or more times.
$            end tag

Update:

As Richard Szalay rightly points out, this is ASCII only (or, more correctly, any encoding scheme where the A-Z, a-z and 0-9 groups are contiguous) and only for the “English” letters.

If you want true internationalized REs (only you know whether that is a requirement), you’ll need to use one of the more appropriate RE engines, such as the PCRE mentioned above, and ensure it’s compiled for Unicode mode. Then you can use “characters” such as \p{L} and \p{N} for letters and numerics respectively. I think the RE in that case would be:

^\p{L}[\pL\pN]*$

but I’m not certain. I’ve never used REs for our internationalized software. See here for more than you ever wanted to know about PCRE.