public static final String regularExpression = "(^[a-zA-Z][0-9_|a-zA-Z]{7,29})";

Actually what's going here.
^  - > means the first character starting with a-z or A-Z

[a-zA-Z] ->  means any character in b/w a-z or A-Z

Next step

[0-9_|a-zA-Z]  -> means any character in 0-9 _ or a-zA-Z
Note ---> | 
this is or symbol its mean either 0-9 or a-zA-Z  





Like 
If(x%2==0 || x%5==0)


now {7,29} means my character range limit

why we use here 7,29 not 8,30 ?

Simply first character takes first digit i.e either a-z or A-Z
means for a-z or A-Z -> 1
 
 then remainning digit is 7 
 so for this reason we put here 7
 
 after putting here 7 than also we decerment 30-1 because first character already takes 1place
 that's why we take word limit 7,29
 
 
 Hope You UnderStand :)