using eregi_replace to replace only substrings -- not the entire string.

I'm trying to update a number in a text file using eregi_replace. The original line is: $registrationLimit = 100; and I'd like to change it to: $registrationLimit = 300; now, 100 appears multiple times throughout the file, so I have to match on the whole string, not just 100. I used the following code: (this may wrap in your mail client) $contents = eregi_replace("registrationLimit = ([0-9]+);","registrationLimit = " . $form_registrationLimit . ";",$contents); which works, but I'd like to find a more elegant way to do this, rather than rebuilding the entire string. I tried: $contents = eregi_replace("registrationLimit = ([0-9]+);",\\1$form_registrationLimit,$contents); but that only produces: $100300 (in other words, it combines the two values and strips out the "registrationLimit = " part...) After reading the manual, I understand why, but I'm hoping there's some similar such method where I can parenthesize a substring in the string pattern and then replace only that substring with my replacement string. Any thoughts? --kurt