Question

okay so I set up this regex to take exponential and give me back doubles but it crashes if I give it less than two

String sc = "2 e 3+       1";

Pattern pattern = Pattern.compile("\\s+");
Matcher matcher = pattern.matcher(sc);
boolean check = matcher.find();
sc = matcher.replaceAll("");
String sc1;
Pattern p = Pattern.compile("[0-9]+[.]?[0-9]*[eE][-+]?[0-9]+");
Matcher m = p.matcher(sc);
m.find();
int starting = m.start(); //where first match starts
int ending = m.end();//where it ends
String scientificDouble = sc.substring(starting, ending);
String replacement = "" + Double.parseDouble(scientificDouble);
sc1 = sc.replace(scientificDouble, replacement); //first time

//this block must go in a loop until getting to the end, i.e. as long as m.find() returns true
m.find();
starting = m.start(); //where next match starts
ending = m.end();//where it ends
scientificDouble = sc.substring(starting, ending);
replacement = "" + Double.parseDouble(scientificDouble);
sc1 = sc1.replace(scientificDouble, replacement);//all other times,

if I give it sc = “2e3 + 1 ” it crashes saying

Exception in thread "main" java.lang.IllegalStateException: No match available
at java.util.regex.Matcher.start(Matcher.java:325)
at StringMan.main(StringMan.java:32)

Answer 1

To add to @Stephen C’s answer, find() will not magically loop the block of code for you. Since you want to execute the same block of code as long as find() returns true, you should use a while loop:

String source = "2e3+1, 2.1e+5, 2.8E-2";
Pattern pattern = Pattern.compile("[0-9]+(\\.[0-9]+)?[eE][\\-\\+]?[0-9]*");
Matcher matcher = pattern.matcher(source);
while (matcher.find()) {
   String match = matcher.group();
   double d = Double.parseDouble(match);
   System.out.println(d);
}

(using regex correctly suggested by @Martijn Courteaux’s answer)

This particular example prints all parsed double to System.out – you can do what you need with them.

Please upvote the cited answers if this is helpful.