Remove HTML comments with Regex, in Javascript

I've got some ugly HTML generated from Word, from which I want to strip all HTML comments.

The HTML looks like this:

<!--[if gte mso 9]><xml> <o:OfficeDocumentSettings> <o:RelyOnVML/> <o:AllowPNG/> </o:OfficeDocumentSettings> </xml><![endif]--><!--[if gte mso 9]><xml> <w:WordDocument> <w:View>Normal</w:View> <w:Zoom>0</w:Zoom> <w:TrackMoves/> <w:TrackFormatting/> <w:HyphenationZone>21</w:HyphenationZone> <w:PunctuationKerning/> <w:ValidateAgainstSchemas/> <w:SaveIfXMLInvalid>false</w:SaveIfXMLInvalid> <w:IgnoreMixedContent>false</w:IgnoreMixedContent> <w:AlwaysShowPlaceholderText>false</w:AlwaysShowPlaceholderText> <w:DoNotPromoteQF/> <w:LidThemeOther>NO-BOK</w:LidThemeOther> <w:LidThemeAsian>X-NONE</w:LidThemeAsian> <w:LidThemeComplexScript>X-NONE</w:LidThemeComplexScript> <w:Compatibility> <w:BreakWrappedTables/> <w:SnapToGridInCell/> <w:WrapTextWithPunct/> <w:UseAsianBreakRules/> <w:DontGrowAutofit/> <w:SplitPgBreakAndParaMark/> <w:EnableOpenTypeKerning/> <w:DontFlipMirrorIndents/> <w:OverrideTableStyleHps/> </w:Compatibility> <m:mathPr> <m:mathFont m:val="Cambria Math"/> <m:brkBin m:val="before"/> <m:brkBinSub m:val="&#45;-"/> <m:smallFrac m:val="off"/> <m:dispDef/> <m:lMargin m:val="0"/> <m:rMargin m:val="0"/> <m:defJc m:val="centerGroup"/> <m:wrapIndent m:val="1440"/> <m:intLim m:val="subSup"/> <m:naryLim m:val="undOvr"/> </m:mathPr></w:WordDocument> </xml><![endif]-->

..and the regex I am using is this one

html = html.replace(/<!--(.*?)-->/gm, "")

But there seems to be no match, the string is unchanged.

What I am missing?

Answers:

Answer

This is based off Aurielle Perlmann's answer, it supports all cases (single-line, multi-line, un-terminated, and nested comments):

/(<!--.*?-->)|(<!--[\S\s]+?-->)|(<!--[\S\s]*?$)/g

https://regex101.com/r/az8Lu6/1

regex101 output

Answer

The regex /<!--[\s\S]*?-->/g should work.

You're going to kill escaping text spans in CDATA blocks.

E.g.

<script><!-- notACommentHere() --></script>

and literal text in formatted code blocks

<xmp>I'm demoing HTML <!-- comments --></xmp>

<textarea><!-- Not a comment either --></textarea>

EDIT:

This also won't prevent new comments from being introduced as in

<!-<!-- A comment -->- not comment text -->

which after one round of that regexp would become

<!-- not comment text -->

If this is a problem, you can escape < that are not part of a comment or tag (complicated to get right) or you can loop and replace as above until the string settles down.


Here's a regex that will match comments including psuedo-comments and unclosed comments per the HTML-5 spec. The CDATA section are only strictly allowed in foreign XML. This suffers the same caveats as above.

var COMMENT_PSEUDO_COMMENT_OR_LT_BANG = new RegExp(
    '<!--[\\s\\S]*?(?:-->)?'
    + '<!---+>?'  // A comment with no body
    + '|<!(?![dD][oO][cC][tT][yY][pP][eE]|\\[CDATA\\[)[^>]*>?'
    + '|<[?][^>]*>?',  // A pseudo-comment
    'g');
Answer

You should use the /s modifier

html = html.replace(/<!--.*?-->/sg, "")

Tested in perl:

use strict;
use warnings;

my $str = 'hello <!--[if gte mso 9]><xml> <o:OfficeDocumentSettings> <o:RelyOnVML/> <o:AllowPNG/> </o:OfficeDocumentSettings> </xml><![endif]--><!--[if gte mso 9]><xml> <w:WordDocument> <w:View>Normal</w:View> <w:Zoom>0</w:Zoom> <w:TrackMoves/> <w:TrackFormatting/> <w:HyphenationZone>21</w:HyphenationZone> <w:PunctuationKerning/> <w:ValidateAgainstSchemas/> <w:SaveIfXMLInvalid>false</w:SaveIfXMLInvalid> <w:IgnoreMixedContent>false</w:IgnoreMixedContent> <w:AlwaysShowPlaceholderText>false</w:AlwaysShowPlaceholderText> <w:DoNotPromoteQF/> <w:LidThemeOther>NO-BOK</w:LidThemeOther> <w:LidThemeAsian>X-NONE</w:LidThemeAsian> <w:LidThemeComplexScript>X-NONE</w:LidThemeComplexScript> <w:Compatibility> <w:BreakWrappedTables/> <w:SnapToGridInCell/> <w:WrapTextWithPunct/> <w:UseAsianBreakRules/> <w:DontGrowAutofit/> <w:SplitPgBreakAndParaMark/> <w:EnableOpenTypeKerning/> <w:DontFlipMirrorIndents/> <w:OverrideTableStyleHps/> </w:Compatibility> <m:mathPr> <m:mathFont m:val="Cambria Math"/> <m:brkBin m:val="before"/> <m:brkBinSub m:val="&#45;-"/> <m:smallFrac m:val="off"/> <m:dispDef/> <m:lMargin m:val="0"/> <m:rMargin m:val="0"/> <m:defJc m:val="centerGroup"/> <m:wrapIndent m:val="1440"/> <m:intLim m:val="subSup"/> <m:naryLim m:val="undOvr"/> </m:mathPr></w:WordDocument> </xml><![endif]-->world!';

$str =~ s/<!--.*?-->//sg;
print $str;

Output:
hello world!

Answer

this works also for multiline - (<!--.*?-->)|(<!--[\w\W\n\s]+?-->)

enter image description here

Answer
html = html.replace("(?s)<!--\\[if(.*?)\\[endif\\] *-->", "")
Answer

const regex = /<!--(.*?)-->/gm;
const str = `You will be able to see this text. <!-- You will not be able to see this text. --> You can even comment out things in <!-- the middle of --> a sentence. <!-- Or you can comment out a large number of lines. --> <div class="example-class"> <!-- Another --> thing you can do is put comments after closing tags, to help you find where a particular element ends. <br> (This can be helpful if you have a lot of nested elements.) </div> <!-- /.example-class -->`;
const subst = ``;

// The substituted value will be contained in the result variable
const result = str.replace(regex, subst);

console.log('Substitution result: ', result);

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