xterm regex definition

[Zenaan Harkness <zen@freedbms.net>]

Is there a way I might determine the full/precise regular expression
syntax for xterm onXclicks resource, without reading the source code?
xterm's regex syntax appears to be not documented. I guess that's a
bug against its man-page??

So I use (the ⌐ is the symbol displayed for a tab character - I'm also
wondering if there's an xterm regex escape for tab, rather than the
CTRL-V then TAB combination):

xterm*on2Clicks:  regex [^<=>|/⌐\n▌▐]+
xterm*on3Clicks:  regex [^=⌐\n▌▐]+
xterm*on4Clicks:  regex [^#$ ][^#$\n]+
xterm*on5Clicks:  line

But for ..4clicks would like something like:
xterm*on4Clicks:  regex [^#$]\ ([^\n]+)\\1
ie, the bracketed expression is the part that I want - would like '\1'
or \\1 to return that "(...)" part of my regex as the to-be-selected
portion of text in my xterm.
ie do back-expressions work in xterm regexes - have I got the syntax
wrong, or are they simply not supported.

TIA
Zenaan