XML Formatter & Validator

About this XML formatter and validator

This tool is an in‑browser XML formatter, pretty printer, and basic XML error checker. It takes raw XML text, validates that it is well‑formed, and then reprints it with consistent indentation so that the structure is easy to read. Everything runs locally in your browser: your XML is not uploaded to a server.

The validator focuses on well‑formedness only. That means it checks whether the XML obeys core syntax rules (matching tags, valid nesting, a single root element, and so on). It does not validate against XML Schema (XSD), DTDs, or other schema languages. If you need those checks, you will still need a separate schema‑aware validator.

How to use the XML formatter

1. Paste your XML into the XML Input box above. You can copy it from a file, an API response, a log entry, or any other source.
2. Run the formatter and validator using the controls in the form (for example a Format or Validate button, depending on your setup).
3. Review the result in the output area. If the XML is well‑formed, you will see a nicely indented, pretty‑printed version of your document.
4. Fix any errors if the tool reports a parser error message instead of formatted XML, then re‑run the formatter until the document is valid.

You can run the formatter repeatedly as you edit your XML. This is useful when debugging integration issues, cleaning up minified XML before code reviews, or preparing examples for documentation.

Before and after: XML pretty printer example

The main benefit of pretty printing is readability. Dense, single‑line XML is hard to scan. Once you format it, the hierarchical structure becomes obvious.

Unformatted XML

Formatted XML output

With indentation, you can immediately see that there is one root element, two children, and several entries. This makes it much easier to reason about the XML structure or spot elements that are missing or misplaced.

What the XML validator checks

This online XML checker validates that your input is a well‑formed XML document. In practice, that involves several core rules:

• There must be exactly one root element that encloses all other elements.
• Elements must be properly nested; tags cannot partially overlap.
• Every start tag must have a matching end tag, unless it is a self‑closing empty‑element tag.
• Attribute names within the same element must be unique.
• Reserved characters such as < and & must be escaped properly inside text nodes or attributes.
• The document must follow the basic XML encoding and character rules for the declared encoding.

If any of these conditions are violated, the browser’s XML parser produces a special error document. This tool detects that and surfaces the error message to you so that you can quickly locate and fix the problem.

How the formatting and validation work

Under the hood, the formatter uses the browser’s built‑in DOM parsing capabilities. When you submit your XML, the script passes the text to a parser that constructs a Document Object Model (DOM). If the parser succeeds, the tool walks the resulting tree of nodes and serializes it back to text with indentation based on depth.

A high‑level way to think about the processing cost is that each node in the XML tree is visited once. If there are N total nodes, the computational complexity is proportional to N. In more formal terms, if an element node has k child nodes with sizes n₁, n₂, …, n_k, the time to process that element can be described as:

Here, c is the constant amount of work needed to serialize the current node (writing its start tag, attributes, end tag, and appropriate whitespace). Because each node is processed once, the overall complexity remains linear in the size of the document for typical inputs.

Validation uses the same parsing step. If the parser encounters invalid syntax, it does not build a normal DOM tree. Instead, it returns an error representation, commonly including a element with human‑readable details about what went wrong and sometimes a line or column number. The tool simply checks for the presence of this error element and, if it exists, displays its text content as an error message instead of pretty‑printed XML.

Interpreting the XML validation results

After running the checker, you will see one of two outcomes:

• Formatted XML output: Your XML is well‑formed. You can copy the result, save it back to a file, or use it in documentation, tests, or API payloads.
• Error message instead of output: The XML is not well‑formed. The message will usually indicate the nature of the error (for example, mismatched tag, opening and ending tag mismatch, or entity not defined), and may include a line and column to guide you.

If you receive an error, look near the indicated location in your original input for unbalanced tags, stray characters, or missing quotes. Once corrected, re‑run the tool to confirm that the document is now valid and properly formatted.

Worked example: finding and fixing an XML error

Consider this small document representing a product catalog:

  
    Coffee mug
    7.95
  
  
    Tea kettle
    29.90
  

The last element is missing its closing slash; it should be . If you paste this into the XML validator and run it, you will see an error message about mismatched tags. The browser notices that the second never closes before the element ends.

After fixing the final line and re‑running the tool, you should get valid, pretty‑printed XML, confirming that the error is resolved.

XML formatter vs. schema‑aware XML validator

This page focuses on pretty printing and basic well‑formedness checks. For some workflows, you may also need full schema validation. The table below compares what this tool does with what a schema‑aware XML validator can do.

Limitations, assumptions, and practical notes

To set expectations clearly, here is what this XML tool does and does not do.

What this tool does

• Formats and pretty prints XML with consistent indentation to improve readability.
• Checks for basic well‑formedness errors using your browser’s XML parser.
• Displays parser error messages when the input is not valid XML.
• Runs entirely in the browser, keeping your XML content local to your device.

What this tool does not do

• It does not validate XML against XSD, DTD, Relax NG, or any other schema language.
• It does not enforce custom business rules beyond what XML syntax itself requires.
• It is not optimized for extremely large XML files; very large documents may be slow or may exceed browser memory limits.
• It does not transform XML (for example using XSLT) or convert XML to JSON.

Assumptions and environment

• The tool assumes a modern browser with support for DOM parsing APIs.
• Encoding is interpreted according to what the browser detects; unusual encodings may behave differently.
• Whitespace and indentation in the output are intended for human readability, not for byte‑perfect round‑tripping in systems that care about exact whitespace.

For most everyday tasks, this in‑browser XML pretty printer and error checker offers a quick, private, and convenient way to clean up XML and confirm that it is structurally sound before you send it to other tools or services.