How do I find out if first character of a string is a number

Question

In Java is there a way to find out if first character of a string is a number   One way is   string startsWith  1     and do the above all the way till 9  but that seems very inefficient

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IN KOTLIN   Suppose that you have a String like this   private val phoneNumber  quot 9121111111 quot   At first you should get the first one   val firstChar phoneNumber slice 0  0   At second you can check the first char that return a Boolean   firstChar isInt      or isFloat

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Character isDigit string charAt 0     Note that this will allow any Unicode digit  not just 0-9  You might prefer   char c   string charAt 0   isDigit    c  gt    0   amp  amp  c  lt    9      Or the slower regex solutions   s substring 0  1  matches    d      or the equivalent s substring 0  1  matches   0-9      However  with any of these methods  you must first be sure that the string isn t empty  If it is  charAt 0  and substring 0  1  will throw a StringIndexOutOfBoundsException  startsWith does not have this problem   To make the entire condition one line and avoid length checks  you can alter the regexes to the following   s matches    d        or the equivalent s matches   0-9        If the condition does not appear in a tight loop in your program  the small performance hit for using regular expressions is not likely to be noticeable

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Regular expressions are very strong but expensive tool  It is valid to use them for checking if the first character is a digit but it is not so elegant    I prefer this way    public boolean isLeadingDigit final String value       final char c   value charAt 0       return  c  gt    0   amp  amp  c  lt    9

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regular expression starts with number- gt    0-9    Pattern pattern   Pattern compile    0-9      Matcher matcher   pattern matcher String    if matcher find      System out println  true

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I just came across this question and thought on contributing with a solution that does not use regex   In my case I use a helper method   public boolean notNumber String input       boolean notNumber   false      try              must not start with a number          SuppressWarnings  unused           double checker   Double valueOf input substring 0 1              catch  Exception e            notNumber   true                       return notNumber      Probably an overkill  but I try to avoid regex whenever I can

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To verify only first letter is number or character -- For number Character isDigit str charAt 0     --return true For character Character isLetter str charAt 0     --return true

[java] How do I find out if first character of a string is a number?

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