linux - grep: Invalid back reference for date expression in tcsh

I googled for the following error, and didn't get any great explanations as to what was going on with grep under tcsh. (Yes, that date expression exists in the log files).

$ grep '2014-07-21' *.log
grep: Invalid back reference

Curiously, if I switch to bash, the command works fine (that is my current work-around). Anyone know what's going on here?

Because someone will ask, here are the results of which and alias:

$ which grep
/bin/grep

$ alias grep
$               # (Nothing)

This is running on RHEL 5.4 (Red Hat).