protected boolean isValidIdentifier(String identifier) {

        // Empty identifiers are invalid
        if (identifier.isEmpty())
            return false;

        // Identifier is invalid if any non-numeric characters are present
        for (int i = 0; i < identifier.length(); i++) {
            if (!Character.isDigit(identifier.charAt(i)))
                return false;
        }

        // Identifier is valid - contains only numeric characters
        return true;

    }
Returns whether the given string is a valid identifier within the JDBC
authentication extension. Invalid identifiers may result in SQL errors
from the underlying database when used in queries.

@param identifier
    The string to check for validity.

@return
    true if the given string is a valid identifier, false otherwise.
extensions/guacamole-auth-jdbc/modules/guacamole-auth-jdbc-base/src/main/java/org/apache/guacamole/auth/jdbc/base/ModeledDirectoryObjectService.java