XML , for Extensible Markup Language, allows users to put tags around their data to describe what pieces of the data mean.

<CATALOG>
  <CD>
    <TITLE>Empire Burlesque</TITLE>
    <ARTIST>Bob Dylan</ARTIST>
    <COUNTRY>USA</COUNTRY>
    <COMPANY>Columbia</COMPANY>
    <PRICE>10.90</PRICE>
    <YEAR>1985</YEAR>
  </CD>
  <CD> ... </CD>
</CATALOG>

We can see that XML provides a hierarchical tree structure for data. The task of checking the validity of an XML file is essentially the same as checking for balanced parentheses; XML simply allows the users to define their own parenthesis names.

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