Thread: validate a String in java Regular Expression

Dear Friends Kindly help me to validate a String in following way

a. If first character is an alphabet, input should be Alpha Numeric 24 characters
b. If first character is a digit, input should be minimum 15 and maximum 16 Numeric characters

I need a regular Expression or Java program example

Thanks in advance

Dear I made the program but not success ,
and thanks for suggestion I will consider for next time
Regards

--- Update ---

String string="PK448345678901aSSSsfry35";

int length=string.length();

if(string.substring(0, 1).matches("[0-9]")){
if(length==15 ||length==16){

System.out.println("First Character is a number..! and Length is "+string.length());
}
else{
throw new InvalidSpeedException("Please Check First Character is Number or check the length");
}
}
if(string.substring(0, 1).matches("[a-zA-Z]")&& matchNum.find()){
if(matcher.find()&& length==24){
System.out.println("First character is Character! and Length is "+string.length());
}
else{
System.out.println("Exception");
throw new InvalidSpeedException("Please Check First is Character or the length");
}
}

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