Right-back Foster has been getting pelters off his Light Blues team-mates in recent weeks for his facial hair.
But the former Aberdeen and Bristol City player has revealed he is reluctant to get rid of it for superstitious reasons.
For the 28-year-old has not lost a game with the Gers since he stopped shaving - and he is keen to prolong that run in the next two games.
Ally McCoist's side take on Raith Rovers in the Ramsdens Cup final at Easter Road on Sunday and then play Dundee United at Ibrox in the William Hill Scottish Cup semi-final six days later.
Foster said: "I don't know when I'll shave it off. I won't be shaving it off if we win on Sunday. If we win I'll need to keep it for Dundee United. We'll see if it's a lucky beard or not.
"It started off because I was being a bit lazy and didn't shave. But I've actually grown to like it. It's the first time I've tried growing a beard. The boys don't mention it at all! Just all the time!
"I did initially think I would keep it until I scored - but I'm not commiting myself to that. That could take a while.
"I'll leave it on for now and if we lose certain games then I might shave it off. All players have little superstitions.
"We got through in the Scottish Cup against Albion Rovers in the cup replay and it's stayed."
He added: "I've got little things that I do before games, but the beard is a new one. I've got back in the team and the beard has been with me.
"I'm doing all right, I don't want to be like Samson, cut off my hair and lose all my powers. I'm enjoying being back in the team and we're all looking forward to a massive week."
Foster played for Rangers in the Champions League against Valencia, Bursaspor and Manchester United during his loan spell at Ibrox in the 2010/11 season.
And he also helped Walter Smith's side to lift the third of three consecutive Scottish titles during that memorable campaign.
But the experienced player is still desperate to help the Gers beat SPFL Championship club Raith and lift the Ramsdens Cup on Sunday.
He said: "It's a huge week for us. You want to win things at Rangers and you are expected to win things here.
"It's an exciting week because it's a cup final on Sunday followed by a massive semi-final against United. It's something all the players are relishing. These two games are very easy to get motivated for.
"I've played in big games. I won a league winner's medal the last time I was here at Rangers. But I haven't won any cups. Winning on Sunday would be right up there in terms of my achievements in Scottish football.
"You come to Rangers to win things and the Ramsdens Cup is no different. It would be a massive victory for the club.
"We want to finish the season on a high because it will give us something to take into next year."