Regex Example: Username Checker

Problem: Check following condition on a username

  • Can contain alphanumeric number and underscore
  • Must not start with a digit
  • Can have length from 8 to 30 characters


Lets generate pattern one by one. Start by checking first character of username which should not be a digit so we have to use range to check start of string.

^([a-zA-Z]) this will check first character of string should be in either in uppercase or lowercase. If you have condition of having uppercase as start of string we have to use like this ^[A-Z].

Now after first character it can have alphanumerc or underscore so we have to use w.Now our expression become like this ^[a-z][A-Z]w (This is relative to Java that’s why i am using double slash).

Now as we have length condition also we have to apply for that also. TO checking first character we have applied regex now left length of string is from 7 to 29 which we app to w using {n,m} now regex become like this  ^[a-zA-Z]w{7,29} . As our pattern is ready we have to apply end of line.

Actual Pattern of username is  : ^[a-zA-Z]w{7,29}$



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