[a-zA-Z]_*[a-zA-Z0-9]*
which I'm aiming for to match :
astring_something;
helloall90
but not :
Astring
_helloall
My regex is protecting me with the identifiers should start with small case letters. But it doesn't work for _ cases. Passing the string :
astring_something;
Is not been properly identified. Its been identified as astring and something leaving out _.
Where I'm making the mistake?
I imagine you want it to start with a letter followed by zero or more letters, numbers or underscores. If so, you need to move the underscore into the second set of characters.
Change:
[a-zA-Z]_*[a-zA-Z0-9]*
To:
[a-zA-Z][a-zA-Z0-9_]*
Or, if it must start with a lowercase letter:
[a-z][a-zA-Z0-9_]*
You're currently matching only a single character before an underscore.
[a-zA-Z]*_[a-zA-Z0-9]*
Whether or not that's what you really want is a different issue; what about things with multiple underscores, for example?
Try
^[a-z]+_*[a-zA-Z0-9]+
Where ...
^[a-z]+
means it must start with one or more lower case letters, followed by ...
_*
zero or more _ characters, followed by ...
[a-zA-Z0-9]*
zero or more alphanumeric characters.
This pattern will match astring_something and helloall90 but will not match _helloall and Astring
