HTML MINIFY RegEx

minify_function.php

$re = '%# Collapse whitespace everywhere but in blacklisted elements.
	(?>             # Match all whitespans other than single space.
	[^\S ]\s*     # Either one [\t\r\n\f\v] and zero or more ws,
	| \s{2,}        # or two or more consecutive-any-whitespace.
	) # Note: The remaining regex consumes no text at all...
	(?=             # Ensure we are not in a blacklist tag.
	[^<]*+        # Either zero or more non-"<" {normal*}
	(?:           # Begin {(special normal*)*} construct
	<           # or a < starting a non-blacklist tag.
	(?!/?(?:textarea|pre|script)\b)
	[^<]*+      # more non-"<" {normal*}
	)*+           # Finish "unrolling-the-loop"
	(?:           # Begin alternation group.
	<           # Either a blacklist start tag.
	(?>textarea|pre|script)\b
			| \z          # or end of file.
	)             # End alternation group.
	)  # If we made it here, we are not in a blacklist tag.
	%Six';

$minified = preg_replace($re, " ", $html_to_minify);

very slow on large amount of HTML

Here the same in JavaScript (ECMAScript Standard) format. Formatted and with explanation.

([^\S ]\s*| \s{2,})(?=[^<]*(?:<(?!\/?(?:textarea|pre|script)\b)[^<]*)*(?:<(textarea|pre|script)\b| \z))

(             # Match all whitespans other than single space.
[^\S ]\s*     # Either one [\t\r\n\f\v] and zero or more ws,
| \s{2,}        # or two or more consecutive-any-whitespace.
) # Note: The remaining regex consumes no text at all...
(?=             # Ensure we are not in a blacklist tag.
[^<]*        # Either zero or more non-"<" {normal*}
(?:           # Begin {(special normal*)*} construct
<           # or a < starting a non-blacklist tag.
(?!\/?(?:textarea|pre|script)\b)
[^<]*      # more non-"<" {normal*}
)*           # Finish "unrolling-the-loop"
(?:           # Begin alternation group.
<           # Either a blacklist start tag.
(textarea|pre|script)\b
	| \z          # or end of file.
)             # End alternation group.
)  # If we made it here, we are not in a blacklist tag.

Also, you forgot a backslash. Here the fixed PHP (PCRE2) version.

(?>[^\S ]\s*| \s{2,})(?=[^<]*+(?:<(?!\/?(?:textarea|pre|script)\b)[^<]*+)*+(?:<(?>textarea|pre|script)\b| \z))

How well does this work? Did you test It?

@pmaojo was working well for long time. yes tested it in had in production