Do not support {n, m} feature.

[cmtheit]

Regex is [a-z][a-z0-9]{0, 10},
Match string is "cmtheit",
Do not match.

[mickjc750]

Testing this on https://regex101.com/
Your Regex appears to be broken by the space after the ,
Instead of:
[a-z][a-z0-9]{0, 10}
try:
[a-z][a-z0-9]{0,10}