Provide help or warnings for syntax in Replace With field

[RaymondLim]

As you can see in the above screenshot, when the user types a replacement text with $1, $_ or $', we should check the regex syntax in Find text field and warn the user if the regex syntax is wrong (or won't work as expected).

[marcelgerber]

Notice $_ and $' won't work in Brackets.
But you're right, it would be cool to have.

[peterflynn]

Reviewed. Low priority @larz0 to think about what kind of guidance & feedback we want to give for replacement in general:

• Should we give syntax guidance about how to insert regexp groups? ($1 vs. \1, how to escape the $, etc.). Some editors have a very fancy UI where you can insert (and drag to rearrange) little placeholder icons in the string so you don't have to think about syntax... pretty sure we don't want anything that elaborate here, but you could imagine a little button that just inserts '$1' into the string, with a tooltip or link to more info from there. Etc.
• Should we give warnings if the user puts $1/etc. in the replacement string but the query isn't regexp mode?
• Should we give warnings if the user appears to be misusing the replacement syntax? (E.g. if we see n they probably meant $n; if we see $n they probably meant $$n; etc.)
• Should we give errors/warnings if the user puts $n where n is out of range? (E.g. in the example above there are zero capturing groups in the regexp, so $1 to reference the 1st capturing group is essentially invalid).

Nominated for 1.0 since we've seen a few people get confused about replacement syntax in general (it's pretty inconsistent across editors), so some kind of usability polish here is probably worthwhile.