A microprocessor controlled thermal printer particularly usable in a hand-held labeler for printing labels on a composite web detects indices on the web and utilizes the detected indices to position the labels relative to the print head. The printer utilizes an improved index detection system that compensates for variations in index size and density and for variations in sensor sensitivity by detecting both the leading and trailing edges of an index and utilizing the distance between the detected leading and trailing edges to determine the length of the index. By dividing the length thus determined by two, and by advancing the web by this amount after the detection of the leading edge of an index, the center of the index can be accurately located, and the web can be accurately registered at any desired position relative to the center of the index.