Ubuntu – Bash: Regular Expressions in substitution

I have a bash script to substitute number values inside a text pattern. Suppose I have a file like this:

word5word
word55word

And I want it to look like this:

word125word
word125word

I have this script to do that:

#!/bin/bash

re='([0-9]|[0-9][0-9])'
while read line
  do
    new_line=${line/"word"$re"word"/"word"125"word"}
    echo "$new_line"
  done < /home/tomak/test.txt

For some reason, the 're' expression is not interpretted correctly and it prints the original lines. I just can't figure out why. It works for a singe digit, i.e. re='[0-9]'.

To formulate the expression, I used information from the Bash Guide for Beginners which states that:

Two regular expressions may be joined by the infix operator "|"; the
resulting regular expression matches any string matching either
subexpression.

But it just doesn't work for me. What am I missing?

Note that I tried to set shopt -s extglob and formulate the expression like [0-9][0-9]? but that didn't work either.

I'm on Ubuntu 14.10 and have the stock bash version 4.3.30. I run the script with bash foo.sh.