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}