定 and 正 don't sound similar in modern Mandarin due to historical phonetic changes. They were more similar
in older Chinese.
Character Evolution
Oracle Bone~1250-1000 BCBronzeMid Western Zhou~900 BCBronzeMid Western Zhou~900 BCBronzeLate Spring and Autumn~500 BCSealWarring States (Chu)475-221 BCSealQin221-206 BCSealShuowen~100 ADClericalSpring and Autumn771-476 BCClericalQin221-206 BCClericalWestern Han202 BC-9 ADClericalWestern Han202 BC-9 ADClericalEastern Han25-220 ADRegularModern