Regular Expressions Matching Simple Patterns Match a single digit character using [0-9] or \d (Java)
Example
[0-9] and \d are equivalent patterns (unless your Regex engine is unicode-aware and \d also matches things like ②). They will both match a single digit character so you can use whichever notation you find more readable.
Create a string of the pattern you wish to match. If using the \d notation, you will need to add a second backslash to escape the first backslash.
String pattern = "\\d";
Create a Pattern object. Pass the pattern string into the compile() method.
Pattern p = Pattern.compile(pattern);
Create a Matcher object. Pass the string you are looking to find the pattern in to the matcher() method. Check to see if the pattern is found.
Matcher m1 = p.matcher("0");
m1.matches(); //will return true
Matcher m2 = p.matcher("5");
m2.matches(); //will return true
Matcher m3 = p.matcher("12345");
m3.matches(); //will return false since your pattern is only for a single integer
