I have this RegEx:
^[a-z0-9](?:[a-z0-9]+)*$
How can I modify this so that a lowercase letter can only appear in my string if there is a number and ONLY after that number?
These are expected matches:
AA
AB
1a
1b
1A
1B
A1A
A1a
AB1
AB2
AB1a
AB2b
These would fail:
Aa
Ab
a1
b1
Ab1
a1a
I would use a positive lookbehind to assert that a digit must precede any lowercase letter:
^(?:[A-Z0-9]|(?<=[0-9])[a-z])*$
As an alternative, a solution without a lookaround could be matching [0-9][a-z] and repeating the outer group 1 or more times to prevent matching an empty string.
^(?:[A-Z0-9]|[0-9][a-z])+$
Explanation
^ Start of string(?: Non capture group
[A-Z0-9] Match either A-Z or 0-9| Or[0-9][a-z] Match 0-9 and then a-z)+ Close group and repeat 1+ times to prevent matching an empty string$ End of string