  We have compiled a sample of 165 radio galaxies from the literature to study
the properties of the extended emission line regions and their interaction with
the radio source over a large range of redshift 0<z<5.2. For each source, we
have collected radio (size, lobe distance ratio and power) and spectroscopic
parameters (luminosity, line width and equivalent width) for the four brightest
UV lines. We also introduce a parameter A_{Ly-alpha} measuring the asymmetry of
the Ly-alpha line. Using these 18 parameters, we examine the statistical
significance of all 153 mutual correlations, and find the following significant
correlations: (i) Ly-alpha asymmetry A_{Ly-alpha} with radio size and redshift,
(ii) line luminosity with radio power, (iii) line luminosities of UV lines with
each other, and (iv) equivalent widths of UV lines with each other. Using
line-ratio diagnostic diagrams, we examine the ionization mechanism of the
extended emission line regions in HzRGs. The high ionization lines seem to
confirm previous results showing that AGN photo-ionization provides the best
fit to the data, but are inconsitent with the CII/CIII ratio, which favour the
highest velocity shock ionization models. We note that the CII line is 5 times
more sensitive to shock ionization than the high ionization UV lines, and show
that a combination of shock and photo-ionization provides a better overall fit
to the integrated spectra of HzRGs. Because most HzRGs have radio sizes <~150
kpc, their integrated spectra might well contain a significant contribution
from shock ionized emission. [abridged]
