An ERJ Captain reported a false localizer capture on approach to 30L at STL. An air carrier aircraft behind reported normal indidcations.

Narrative

During the approach phase to STL we were given a heading to intercept the localizer for the ILS 30L. While we were on the heading (340; intercepting from the south via the QBall arrival) we captured a 'false' localizer twice before intercepting the proper one. We knew it was an issue when the FMS display indicated our distance as approximately 12 miles from the approach course and then again about 5 miles from the approach course. The local cities (we could see visually straight down) we were over were around Dupo; Illinois and then again over Sauget; Illinois. We reported it to ATC and they queried an air carrier following us. They reported nothing unusual. I have seen this happen once before on a visual approach but never in IMC conditions. It is quite unsafe and would easily fool someone who is not intimately familiar with the area and the approaches at STL. Both times the aircraft was taken out of the NAV mode to keep it from following the incorrect localizer. I have heard from other pilots that they have experienced this as well. I cannot determine if it is a problem with our avionics on the E145. This should be investigated by the FAA. A clearance for the approach closer to the actual localizer may prevent further 'false captures.'