Java library to check whether a String contains a number *without* exceptions

Question

I\'m looking for a method that returns a boolean if the String it is passed is a valid number (e.g. \"123.55e-9\", \"-333,556\"). I don\'t want to just do:

Answer 1

The exact regular expression is specified in the Javadocs for Double.valueOf(String).

To avoid calling this method on an invalid string and having a NumberFormatException be thrown, the regular expression below can be used to screen the input string:

final String Digits     = "(\\p{Digit}+)";
final String HexDigits  = "(\\p{XDigit}+)";
// an exponent is 'e' or 'E' followed by an optionally 
// signed decimal integer.
final String Exp        = "[eE][+-]?"+Digits;
final String fpRegex    =
       ("[\\x00-\\x20]*"+  // Optional leading "whitespace"
        "[+-]?(" + // Optional sign character
        "NaN|" +           // "NaN" string
        "Infinity|" +      // "Infinity" string

        // A decimal floating-point string representing a finite positive
        // number without a leading sign has at most five basic pieces:
        // Digits . Digits ExponentPart FloatTypeSuffix
        // 
        // Since this method allows integer-only strings as input
        // in addition to strings of floating-point literals, the
        // two sub-patterns below are simplifications of the grammar
        // productions from the Java Language Specification, 2nd 
        // edition, section 3.10.2.

        // Digits ._opt Digits_opt ExponentPart_opt FloatTypeSuffix_opt
        "((("+Digits+"(\\.)?("+Digits+"?)("+Exp+")?)|"+

        // . Digits ExponentPart_opt FloatTypeSuffix_opt
        "(\\.("+Digits+")("+Exp+")?)|"+

        // Hexadecimal strings
        "((" +
        // 0[xX] HexDigits ._opt BinaryExponent FloatTypeSuffix_opt
        "(0[xX]" + HexDigits + "(\\.)?)|" +

        // 0[xX] HexDigits_opt . HexDigits BinaryExponent FloatTypeSuffix_opt
        "(0[xX]" + HexDigits + "?(\\.)" + HexDigits + ")" +

        ")[pP][+-]?" + Digits + "))" +
        "[fFdD]?))" +
        "[\\x00-\\x20]*"); // Optional trailing "whitespace"
       
if (Pattern.matches(fpRegex, myString))
    Double.valueOf(myString); // Will not throw NumberFormatException
else {
    // Perform suitable alternative action
}