An XML Schema XSD Definition to Prevent Leading Zeros in Integers

The XML Schema specification^1^^ provides several handy data types. For example, _xs:positiveInteger_^2^^ produces "the standard mathematical concept of the positive integer numbers."

Well, mostly.

There's a hidden gotcha in positiveInteger. It allows an anathema. _Leading zeros_.

For example, given the definition:

Code

<xs:element name="node">
  <xs:complexType>
    <xs:attribute name="number" type="xs:positiveInteger"/>
  </xs:complexType>
</xs:element>

These are all valid^3^^:

Code

<node number="1"/>
<node number="100"/>
<node number="8675309"/>
<node number="007"/>

That last one can cause all kinds of havoc.

When leading zeros are involved in data feeds, they have to be treated as either a string (to maintain the zeros) or converted into an actual integer. Given a system of any size or longevity, the likelihood of different processes making opposing choices approaches 100%. Subtle, super-annoying bugs are born. Ones that take a surprisingly large amount of time to fix^4^^.

Thankfully, XML Schema is robust enough that we can define data types that prohibit leading zeros. For example:

Code

<xs:simpleType name="_positive_integer_without_leading_zeros">
  <xs:restriction base="xs:positiveInteger">
    <xs:pattern value="[123456789]\d*"/>
  </xs:restriction>
</xs:simpleType>

<xs:element name="node">
  <xs:complexType>
    <xs:attribute name="number" type="_positive_integer_without_leading_zeros"/>
  </xs:complexType>
</xs:element>

This works by using a Regular Expression^5^^ to enforce the data format. It's a little easier to understand by breaking pattern's `value` into two parts. First:

[123456789]

Anything inside square brackets identifies possible values for a single character. So, `[123456789]` at the start of the pattern value means the first character must be either: 1, 2, 3, 4, 5, 6, 7, 8, or 9. The lack of zero means anything starting with a "0" won't match and will therefore be rejected as invalid.

The second part of the pattern is:

\d*

A `\d` (without the `*`) tells the pattern matcher to look for any single digit. If it was by itself, it would mean there always has to be a second character and that character must be a digit. The `*` modifies `\d` to allow "zero or more" digits.

If the data being matched is a single character, the `\d*` has no real effect. If there are two or more characters, it enforces the restriction that every character from the second until the end must be a digit. Unlike the earlier `[123456789]`, the `\d` pattern includes all possible digits, including zero.

Combined, the `[123456789]\d*` pattern produces the desired behavior. These actual integers all pass the validation test:

Code

<node number="1"/>
<node number="100"/>
<node number="8675309"/>

But this one's accurately rejected as invalid:

Code

<node number="007"/>

This little snippet is now safe from those pesky little leading zeros sneaking in.

Count yourself lucky if you've never had to deal with leading zeros. If you want to avoid them in future XML use this type of custom data type instead of _xs:positiveInteger_.

_Notes_

1. This technique works equally well in XML Schema 1.0 and 1.1.

2. The `xs:positiveInteger` data type allows "+" at the start of the number (e.g. "+8675309"). The definition above doesn't. I built it to deal with unique IDs from a database. None of which contain the "+". Changing the pattern value to `\+?[123456789]\d*` would accommodate the plus if you need it. Other variations are left as exercises for the reader.

_Footnotes_

1. Official XML Schema Documentation

2. xs:positiveInteger details

3. While not all validators find the same things, the "007" string was a valid `xs:positiveInteger_` value in Saxon-EE 9.6.0.5, LIBXML, and Xerces running in oXygen XML Editor. Speaking of which, if you do any XML work at all and don't know about oXygen, you should check it out. It's expensive but totally worth it.

4. This doesn't even begin to get into what happens when everyone agrees that strings are the way to go but you run out of numbers and need to add a new zero to the front.

5. Regular Expressions - a sequence of characters that define a search pattern - the heart and soul of text processing for an old Perl coder like me.