in core/src/main/java/org/apache/jsieve/comparators/AsciiNumeric.java [58:67]
private BigInteger toInteger(final String value) {
int i;
for (i = 0; i < value.length(); i++) {
final char next = value.charAt(i);
if (!isDigit(next)) {
break;
}
}
return new BigInteger(value.substring(0, i));
}