Adding a note that a few English Wikipedian's have discovered this limitation and would like to see this improved.

https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)#Search_intitle:_doesn.27t_work_right_with_quoted_strings_that_include_a_space

I don't think this would be particularly hard to implement, all the functionality already exists. We need to add the appropriate sub-fields to title and adjust the intitle: keyword to swap between term matching and regex matching the same as insource: does today.

In T156474#3713557, @EBernhardson wrote:

I don't think this would be particularly hard to implement, all the functionality already exists. We need to add the appropriate sub-fields to title and adjust the intitle: keyword to swap between term matching and regex matching the same as insource: does today.

Just as a reminder, you left a comment in January at T156510 about this task:

Regex, as suggested above, might be a reasonable way forward there but would need some consideration as it seems possible our regex acceleration wouldn't be as effective on short fields such as title.

Certainly it is possible the shorter field will allow for significantly less filtering prior to running the regex. The acceleration phase basically extract sets of trigrams (three sequential characters) that must be in the searched content from the regex and then look for documents containing those trigrams as a first pass filter. This generally reduces the number of articles we need to run the regex on significantly. I think it is worth keeping in mind, and evaluating.

And just as a side note for implementation, this certainly needs to be applied to both the title field and redirect.title

This was also asked here: https://www.mediawiki.org/wiki/Topic:U86odd2qnp5d78dp

In T156474#3997771, @debt wrote:

This was also asked here: https://www.mediawiki.org/wiki/Topic:U86odd2qnp5d78dp

Yes and the reason I asked was https://commons.wikimedia.org/wiki/Category:Files_with_IMG-date-WA_in_filename. I created it without Cirrus.

Change 413896 had a related patch set uploaded (by EBernhardson; owner: EBernhardson):
[mediawiki/extensions/CirrusSearch@master] Support regex for intitle keyword

https://gerrit.wikimedia.org/r/413896

Don't forget to update the documentation! Is that a separate task?

Yes, but since this keyword will fail before all wikis are reindexed I think we should postpone any doc addition until the reindex is done. A task for adding it to the doc is good idea.

Change 413896 merged by jenkins-bot:
[mediawiki/extensions/CirrusSearch@master] Support regex for intitle keyword

https://gerrit.wikimedia.org/r/413896

In T156474#4006373, @dcausse wrote:

Yes, but since this keyword will fail before all wikis are reindexed I think we should postpone any doc addition until the reindex is done. A task for adding it to the doc is good idea.

@debt Since you closed the subtask, Help:CirrusSearch does not document it yet. :)

In T156474#4101045, @Izno wrote:

In T156474#4006373, @dcausse wrote:

Yes, but since this keyword will fail before all wikis are reindexed I think we should postpone any doc addition until the reindex is done. A task for adding it to the doc is good idea.

@debt Since you closed the subtask, Help:CirrusSearch does not document it yet. :)

Ah yes, thanks for the reminder! I've created T191340 for this documentation task.