TYPO3 Core

I'd prefer to adjust the documentation in this case, as I consider spaces === blank.

Please use ifEmpty, if you want to consider spaces === blank. It is documented, that ifEmpty will trimm the values. That's the big difference between ifEmpty and ifBlank.

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/56305

Patch set 1 for branch TYPO3_8-7 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/56320

Stefan, I see you dilemma but documentation is correct here. It's mentioned both functions ifEmpty and ifBlank are trimming the value. So currently there seems not be a way for a such a case and you can't make a change without breaking current functionality.

fEmpty:
If the trimmed content is empty at this point, the content is loaded with "ifEmpty". Zeros are treated as empty values!

ifBlank:
Same as "ifEmpty" but the check is done using strlen().

My suggestion is providing a new function e.g. ifEmptyNoTrim or ifEmpty2.

I have made a PR to the documentation see https://github.com/TYPO3-Documentation/TYPO3CMS-Reference-Typoscript/pull/148

You can use this TypoScript to workaround the implementation using trim():

# Default PAGE object:
page = PAGE

page.123 = TEXT
page.123.value = blue, green,  , red,,white
page.123.preIfEmptyListNum = 2

page.123.stdWrap.setContentToCurrent = 1
page.123.stdWrap.override = pink
page.123.stdWrap.override {
  if.isFalse.current = 1
  if.isFalse.replacement {
    10 {
      search = /[\s0]/
      replace = 1
      useRegExp = 1
    }
  }
}

page.123.wrap = <p>"|"</p>

page.124 < page.123
page.124.value = blue, green, SOMETHING , red,,white
page.125 < page.123
page.125.value = blue, green,0, red,,white
page.126 < page.123
page.126.value = blue, green,, red,,white

Output is

<p>"  "</p>
<p>" SOMETHING "</p>
<p>"0"</p>
<p>"pink"</p>

Cool!