[TYPO3] HELP: sr_feuser_register restrict username field to a-z

[Cray Flatline]

I have installed sr_feuser_register and all works well. But I want to 
restrict username field (i.e. login name) to A-Z,a-z,0-9. I found in 
core reference what i can set in TCA that field will evaluate to 
"alphanum_x" - but can't undestand how i can implement it.

May be somebody know a any way to solve this question. Please help me.
Thanks a lot. (:


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W BR, Cray Flatline