regex - VB.NET & simple Regular Expression -

i've got extremely simple regular expression contained in subroutine:

private sub txtsubject_leave() handles txtsubject.leave     dim pattern string = "^[a-z0-9]*"     if not regex.ismatch(txtsubject.text, pattern)         msgbox("invalid subject")         txtsubject.focus()     end if end sub 

i'm sure it's going simple every string try erroneous wont return error message, doing wrong?

^[a-z0-9]* 

matches 0 or more alphanumeric characters @ start of string trying match. 0 part important here, because if first character of string space, 0 'match' , pass test. if want make sure first character alphanumeric, use quantifier, +:

^[a-z0-9]+ 

if want make sure string contains alphanumeric characters, use end of line anchor well:

^[a-z0-9]+$ 

in particular situation, if want allow empty string, can use *:

^[a-z0-9]*$ 

this 1 not allow non-alphanumeric characters either.