The Go XML Formatter, xmlfmt, will format the XML string in a readable way.

package main

import "github.com/go-xmlfmt/xmlfmt"

func main() {
	xml1 := `<root><this><is>a</is><test /><message><!-- with comment --><org><cn>Some org-or-other</cn><ph>Wouldnt you like to know</ph></org><contact><fn>Pat</fn><ln>Califia</ln></contact></message></this></root>`
	x := xmlfmt.FormatXML(xml1, "\t", "  ")
	print(x)

	// If the XML Comments have nested tags in them
	xml1 = `<book> <author>Fred</author>
<!--
<price>20</price><currency>USD</currency>
-->
 <isbn>23456</isbn> </book>`
	x = xmlfmt.FormatXML(xml1, "", "  ", true)
	print(x)
}

Output:

	<root>
	  <this>
	    <is>a</is>
	    <test />
	    <message>
	      <!-- with comment -->
	      <org>
	        <cn>Some org-or-other</cn>
	        <ph>Wouldnt you like to know</ph>
	      </org>
	      <contact>
	        <fn>Pat</fn>
	        <ln>Califia</ln>
	      </contact>
	    </message>
	  </this>
	</root>


<book>
  <author>Fred</author>
  <!-- <price>20</price><currency>USD</currency> -->
  <isbn>23456</isbn>
</book>

There is no XML decoding and encoding involved, only pure regular expression matching and replacing. So it is much faster than going through decoding and encoding procedures. Moreover, the exact XML source string is preserved, instead of being changed by the encoder. This is why this package exists in the first place.

Note that

• the default line ending is handled by the package automatically now. For Windows it's CRLF, and standard for anywhere else. No need to change the default line ending now.
• the case of XML comments nested within XML comments is not supported. Please avoid them or use any other tools to correct them before using this package.
• don't turn on the nestedTagsInComments parameter blindly, as the code has become 10+ times more complicated because of it.

To use it on command line, check out xmlfmt:

$ xmlfmt -V
xmlfmt - XML Formatter
Copyright (C) 2016-2022, Antonio Sun

The xmlfmt will format the XML string without rewriting the document

Built on 2022-02-06
Version 1.1.1

$ xmlfmt
the required flag `-f, --file' was not specified

Usage:
  xmlfmt [OPTIONS]

Application Options:
  -f, --file=    The xml file to read from (or "-" for stdin) [$XMLFMT_FILEI]
  -p, --prefix=  Each element begins on a new line and this prefix [$XMLFMT_PREFIX]
  -i, --indent=  Indent string for nested elements (default:   ) [$XMLFMT_INDENT]
  -n, --nested   Nested tags in comments [$XMLFMT_NESTED]
  -v, --verbose  Verbose mode (Multiple -v options increase the verbosity)
  -V, --version  Show program version and exit

Help Options:
  -h, --help     Show this help message


$ curl -sL https://pastebin.com/raw/z3euQ5PR | xmlfmt -f -

<root>
  <this>
    <is>a</is>
    <test />
    <message>
      <!-- with comment -->
      <org>
        <cn>Some org-or-other</cn>
        <ph>Wouldnt you like to know</ph>
      </org>
      <contact>
        <fn>Pat</fn>
        <ln>Califia</ln>
      </contact>
    </message>
  </this>
</root>

$ curl -sL https://pastebin.com/raw/Zs0qy0qz | tee /tmp/xmlfmt.xml | xmlfmt -f - -n

<book>
  <author>Fred</author>
  <!-- <price>20</price><currency>USD</currency> -->
  <isbn>23456</isbn>
</book>

$ XMLFMT_NESTED=true XMLFMT_PREFIX='|' xmlfmt -f /tmp/xmlfmt.xml

|
|<book>
|  <author>Fred</author>
|  <!-- <price>20</price><currency>USD</currency> -->
|  <isbn>23456</isbn>
|</book>

The Go XML Formatter is not called XML Beautifier because the result is not exactly as what people would expect -- most of the closing tags stays on the same line, just as shown above. Having been looking at the result and thinking over it, I now think it is actually a better way to present it, as those closing tags on the same line are better stay that way in my opinion. I.e.,

When it comes to very big XML strings, which is what I’m dealing every day, saving spaces by not allowing those closing tags taking extra lines is plus instead of negative to me.

To format it “properly”, i.e., as what people would normally see, is very hard using pure regular expression. In fact, according to Sam Whited from the go-nuts mlist,

Regular expression is, well, regular. This means that they can parse regular grammars, but can't parse context free grammars (like XML). It is actually impossible to use a regex to do this task; it will always be fragile, unfortunately.

So if the output format is so important to you, then unfortunately you have to go through decoding and encoding procedures. But there are some drawbacks as well, as put by James McGill, in https://stackoverflow.com/questions/21117161, besides such method being slow:

I like this solution, but am still in search of a Golang XML formatter/prettyprinter that doesn't rewrite the document (other than formatting whitespace). Marshalling or using the Encoder will change namespace declarations.

For example an element like "< ns1:Element />" will be translated to something like '< Element xmlns="https://bla...bla/ns1" >< /Element >' which seems harmless enough except when the intent is to not alter the xml other than formatting. -- James McGill Nov 12 '15

Using Sam's code as an example,

https://play.golang.org/p/JUqQY3WpW5

The above code formats the following XML

<soapenv:Envelope xmlns:soapenv="https://schemas.xmlsoap.org/soap/envelope/"
  xmlns:ns="https://example.com/ns">
   <soapenv:Header/>
   <soapenv:Body>
     <ns:request>
      <ns:customer>
       <ns:id>123</ns:id>
       <ns:name type="NCHZ">John Brown</ns:name>
      </ns:customer>
     </ns:request>
   </soapenv:Body>
</soapenv:Envelope>

into this:

<Envelope xmlns="https://schemas.xmlsoap.org/soap/envelope/" xmlns:_xmlns="xmlns" _xmlns:soapenv="https://schemas.xmlsoap.org/soap/envelope/" _xmlns:ns="https://example.com/ns">
 <Header xmlns="https://schemas.xmlsoap.org/soap/envelope/"></Header>
 <Body xmlns="https://schemas.xmlsoap.org/soap/envelope/">
  <request xmlns="https://example.com/ns">
   <customer xmlns="https://example.com/ns">
    <id xmlns="https://example.com/ns">123</id>
    <name xmlns="https://example.com/ns" type="NCHZ">John Brown</name>
   </customer>
  </request>
 </Body>
</Envelope>

I know they are syntactically the same, however the problem is that they look totally different.

That's why there is this package, an XML Beautifier that doesn't rewrite the document.

The credit goes to diotalevi from his post at https://www.perlmonks.org/?node_id=261292.

However, it does not work for all cases. For example,

$ echo '<Envelope xmlns=https://schemas.xmlsoap.org/soap/envelope/ xmlns:_xmlns=xmlns _xmlns:soapenv=https://schemas.xmlsoap.org/soap/envelope/ _xmlns:ns=https://example.com/ns><Header xmlns=https://schemas.xmlsoap.org/soap/envelope/></Header><Body xmlns=https://schemas.xmlsoap.org/soap/envelope/><request xmlns=https://example.com/ns><customer xmlns=https://example.com/ns><id xmlns=https://example.com/ns>123</id><name xmlns=https://example.com/ns type=NCHZ>John Brown</name></customer></request></Body></Envelope>' | perl -pe 's/(?<=>)\s+(?=<)//g; s(<(/?)([^/>]+)(/?)>\s*(?=(</?))?)($indent+=$3?0:$1?-1:1;"<$1$2$3>".($1&&($4 eq"</")?"\n".("  "x$indent):$4?"\n".("  "x$indent):""))ge'

<Envelope xmlns=https://schemas.xmlsoap.org/soap/envelope/ xmlns:_xmlns=xmlns _xmlns:soapenv=https://schemas.xmlsoap.org/soap/envelope/ _xmlns:ns=https://example.com/ns><Header xmlns=https://schemas.xmlsoap.org/soap/envelope/></Header>
<Body xmlns=https://schemas.xmlsoap.org/soap/envelope/><request xmlns=https://example.com/ns><customer xmlns=https://example.com/ns><id xmlns=https://example.com/ns>123</id>
<name xmlns=https://example.com/ns type=NCHZ>John Brown</name>
</customer>
</request>
</Body>
</Envelope>

I simplified the algorithm, and now it should work for all cases:

echo '<Envelope xmlns="https://schemas.xmlsoap.org/soap/envelope/" xmlns:_xmlns="xmlns" _xmlns:soapenv="https://schemas.xmlsoap.org/soap/envelope/" _xmlns:ns="https://example.com/ns"><Header xmlns="https://schemas.xmlsoap.org/soap/envelope/"></Header><Body xmlns="https://schemas.xmlsoap.org/soap/envelope/"><request xmlns="https://example.com/ns"><customer xmlns="https://example.com/ns"><id xmlns="https://example.com/ns">123</id><name xmlns="https://example.com/ns" type="NCHZ">John Brown</name></customer></request></Body></Envelope>' | perl -pe 's/(?<=>)\s+(?=<)//g; s(<(/?)([^>]+)(/?)>)($indent+=$3?0:$1?-1:1;"<$1$2$3>"."\n".("  "x$indent))ge'

<Envelope xmlns="https://schemas.xmlsoap.org/soap/envelope/" xmlns:_xmlns="xmlns" _xmlns:soapenv="https://schemas.xmlsoap.org/soap/envelope/" _xmlns:ns="https://example.com/ns">
  <Header xmlns="https://schemas.xmlsoap.org/soap/envelope/">
    </Header>
  <Body xmlns="https://schemas.xmlsoap.org/soap/envelope/">
    <request xmlns="https://example.com/ns">
      <customer xmlns="https://example.com/ns">
        <id xmlns="https://example.com/ns">
          123</id>
        <name xmlns="https://example.com/ns" type="NCHZ">
          John Brown</name>
        </customer>
      </request>
    </Body>
  </Envelope>

This package is a direct translate from above Perl code into Go, then further enhanced by @ruandao and @chenbihao.