[PHP-DB] problem with regular expression

[Jennifer Arcino Demeterio]

i have a script to identify if the input is a combination of letters and numbers 
 
if (eregi("[a-zA-Z0-9][0-9]+[a-zA-Z][0-9a-zA-Z]",$fieldvalue)) 
print "alphanumeric"
} else {
not an alphanumeric
} 

my problem is, when the string ends with a number, for example hello123, it says "not 
an alpanumeric" ... it works fine when the number is placed before the letters or in 
between letters ... why is that? 

i would really appreciate your help, thanks very much

:)