\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