Range out of order in character class

I'm getting this odd error in the preg_match() function:

Warning: preg_match(): Compilation failed: range out of order in character class at offset 54

The line which is causing this is:

preg_match("/<!--GSM\sPER\sNUMBER\s-\s$gsmNumber\s-\sSTART-->(.*)<!--GSM\sPER\sNUMBER\s-\s$gsmNumber\s-\sEND-->/s", $fileData, $matches);

What this regular expression does is parse an HTML file, extracting only the part between:

<!--GSM PER NUMBER - 5550101 - START-->

and:

<!--GSM PER NUMBER - 5550101 - END-->

Do you have a hint about what could be causing this error?