Home > Articles > Web Services > XML

XML Schema Patterns

  • Print
  • + Share This
This chapter is from the book

This chapter is from the book

15.4 Quantifiers and Quantities

By default an atom represents a fragment of the pattern that must occur in a value and may not repeat:

<pattern value="a" />

  <Code>a</Code>   <!-- OK -->
  <Code></Code>    <!-- ERROR ('a' must be present) -->
  <Code>aaa</Code> <!-- ERROR ('a' must not repeat) -->

The expression 'abc' specifies that there must be one 'a' character, followed by one 'b' character, followed by one 'c' character. The values 'ab', 'bc', and 'abcc' would not match this pattern.

But it is possible to state that an atom is optional or repeatable, or even to specify an allowed range of occurrences. The pattern language achieves this by allowing quantifier symbols to be placed after the atoms they relate to. The symbols '?', '+' and '*' are used for this purpose (and have meanings that will be unsurprising to those familiar with DTD content models). Alternatively, a quantity allows any number of occurrences to be precisely specified.

Optional quantifier

The '?' quantifier indicates that the atom before it is optional. For example:


Legal values in this case include 'abc' and 'ac'.

Note that it is possible to have two identical optional tokens in sequence, such as 'a?a?b'. This is because, unlike the case with DTD and schema element models, look-ahead parsing is permitted. This means the value 'ab' can be matched to this pattern, as can 'aab' (and just 'b'), without causing any problems for the parser. The level of violence and strength of language in a TV program could be indicated with star ratings, '*' (the minimum), '**', '***', '****' or '*****' (the five-star maximum), but perhaps using the letter 's' to represent each star (asterisks cannot be used without further complications that are explained later):

<pattern value="ss?s?s?s?" />

  <Ratings Violence="ssss" StrongLanguage="ss" />

Repeatable quantifier

The '+' quantifier signifies that the atom is repeatable. The atom must be present, but any number of further occurences are allowed. For example:


Legal values in this case include 'abc' and 'abbbbbbbbbc', but 'ac' would not be valid.

It is not ambiguous to create patterns such as 'b+b+' (though it would be pointless). The parser would not need to match a particular 'b' character to one atom or the other (except for the first and last 'b' in the sequence).

Optional and repeatable quantifier

The '*' quantifier indicates that the atom is both optional and repeatable. This could be seen to be functionaly equivalent to '?+' if such combinations were legal:


This expression makes the 'b' atom optional and repeatable so legal values include 'ac', 'abc' and 'abbbbbbc'.

Again, it is not ambiguous to create patterns such as 'b*z?b*'. If the 'z' atom is absent, no attempt is made to decide whether a particular 'b' atom belongs to the first part of the pattern or to the last part.

Greedy quantifiers and backtracking

When a single atom can be matched to multiple characters in a value, matching patterns to values can become quite complex, including multiple interpretations where only one of the possible interpretations would successfully match. The perceived issue here is one of 'greed.' Consider the pattern 'a+b?a' (a series of 'a' characters or a series of 'a' characters followed by a single 'b' character and a single 'a' character) and an attempt to validate the value 'aaa' against it. There are two possible interpretations of the pattern, and one of them would not report a match with the value.

In the first scenario, the first atom, 'a+', could reasonably match the entire value (it could be greedy). But the remainder of the pattern, 'b?a', could not then be matched to anything, so the value would be deemed to be invalid (the missing character 'b' is not a problem here, because it is optional, but the missing additional 'a' character would trigger a failed match).

In the second scenario, the initial atom is only matched to the first two characters of the value instead of all three. The final atom of the pattern, 'a', could then be successfully matched with the final 'a' of the value.

A successful match should be reported if either of the interpretations is applicable to the value. In fact, a pattern is matched to a value first by an attempt at the greedy approach, then, if this fails to match the value, by attempts at less greedy interpretations until (hopefully) a successful match can made.

Readers familiar with the use of expression languages to find text strings should note that in XML an expression is always expected to apply to the content of an entire element or to a complete attribute value. Hence, there need be no concern that expressions could be crafted that might inadvertently (through sheer greed) find a false match spanning two real instances of a value (and everything between) within a single line of text.

Complex example

The following example includes all three quantifiers, and all the following Code elements are valid according to this pattern:

<pattern value="a+b?c*" />



A quantity is a more finely tuned instrument for specifying occurrence options than the qualifiers described above. Instead of a single symbol, such as '+', a quantity involves either one or two integer values enclosed by curly braces ('{' and '}').

The simplest form of quantity involves a single integer value. This value specifies how many times the atom must occur. For example:


This pattern specifies that the value must be 'abbbc'.

A quantity range involves two values, separated by a comma. The first value indicates the minimum number of occurrences allowed, and the second value indicates the maximum number of occurrences allowed. For example:


This pattern specifies that the value must be 'abbbc', 'abbbbc', or 'abbbbbc'.

It is also possible to specify just a minimum number of occurrences. If the second value is absent but the comma is still present, then only a minimum is being specified. The following pattern allows for everything from 'abbc' to 'abbbbbbbbbc' and beyond:


Note that it is not possible to specify just a maximum number of repetitions in this way. It is always necessary to supply a minimum value. However, a minimum value of '0' is allowed, so '{0,55}' achieves the aim of specifying a maximum of '55'.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020