How to check if a string contains only digits in Java

Question

In Java for String class there is a method called matches  how to use this method to check if my string is having only digits using regular expression  I tried with below examples  but both of them returned me false as result   String regex     0-9    String data    23343453   System out println data matches regex        String regex      0-9    String data    23343453   System out println data matches regex

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You can also use NumberUtil isNumber String str  from Apache Commons

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Try  String regex     0-9       or  String regex      d      As per Java regular expressions  the    means  one or more times  and  d means  a digit    Note  the  double backslash  is an escape sequence to get a single backslash - therefore    d in a java String gives you the actual result   d  References    Java Regular Expressions Java Character Escape Sequences     Edit  due to some confusion in other answers  I am writing a test case and will explain some more things in detail   Firstly  if you are in doubt about the correctness of this solution  or others   please run this test case   String regex      d        positive test cases  should all be  true  System out println  1  matches regex    System out println  12345  matches regex    System out println  123456789  matches regex        negative test cases  should all be  false  System out println    matches regex    System out println  foo  matches regex    System out println  aa123bb  matches regex      Question 1      Isn t it necessary to add   and   to the regex  so it won t match  aa123bb      No  In java  the matches method  which was specified in the question  matches a complete string  not fragments  In other words  it is not necessary to use    d    even though it is also correct   Please see the last negative test case   Please note that if you use an online  regex checker  then this may behave differently  To match fragments of a string in Java  you can use the find method instead  described in detail here   Difference between matches   and find   in Java Regex  Question 2      Won t this regex also match the empty string          No  A regex   d  would match the empty string  but   d  does not  The star   means zero or more  whereas the plus   means one or more  Please see the first negative test case   Question 3     Isn t it faster to compile a regex Pattern    Yes  It is indeed faster to compile a regex Pattern once  rather than on every invocation of matches  and so if performance implications are important then a Pattern can be compiled and used like this   Pattern pattern   Pattern compile regex   System out println pattern matcher  1   matches     System out println pattern matcher  12345   matches     System out println pattern matcher  123456789   matches

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You must allow for more than a digit  the   sign  as in   String regex     0-9      String data    23343453    System out println data matches regex

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Try this part of code   void containsOnlyNumbers String str        try           Integer num   Integer valueOf str           System out println  is a number          catch  NumberFormatException e               TODO  handle exception         System out println  is not a number

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Long parseLong data    and catch exception  it handles minus sign    Although the number of digits is limited this actually creates a variable of the data which can be used  which is  I would imagine  the most common use-case

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Using regular expressions is costly in terms of performance  Trying to parse string as a long value is inefficient and unreliable  and may be not what you need   What I suggest is to simply check if each character is a digit  what can be efficiently done using Java 8 lambda expressions   boolean isNumeric   someString chars   allMatch x - gt  Character isDigit x

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According to Oracle s Java Documentation   private static final Pattern NUMBER PATTERN   Pattern compile              x00-  x20    -   NaN Infinity       p Digit             p Digit                    eE   -     p Digit               p Digit      eE   -     p Digit                       0 xX    p XDigit            0 xX    p XDigit            p XDigit                   pP   -     p Digit      fFdD       x00-  x20      boolean isNumber String s   return NUMBER PATTERN matcher s  matches

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We can use either Pattern compile   0-9    0-9     or Pattern compile    d    d     They have the same meaning   the pattern  0-9  means digit  The same as   d       means it appears more times      for integer or float   Try following code   import java util regex Pattern       public class PatternSample            public boolean containNumbersOnly String source               boolean result   false              Pattern pattern   Pattern compile   0-9    0-9        correct pattern for both float and integer              pattern   Pattern compile    d    d       correct pattern for both float and integer               result   pattern matcher source  matches                if result                   System out println        source             is a number                 else                 System out println        source             is a String                return result                     public static void main String   args               PatternSample obj   new PatternSample                obj containNumbersOnly  123456 a                obj containNumbersOnly  123456                 obj containNumbersOnly  123456                obj containNumbersOnly  0123456 0                obj containNumbersOnly  0123456a 0                       Output    123456 a  is a String  123456   is a String  123456  is a number  0123456 0  is a number  0123456a 0  is a String

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One more solution  that hasn t been posted  yet   String regex      p Digit        uses POSIX character class

[java] How to check if a string contains only digits in Java

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