Florida Gardening: Unknown "Weed" Help!, 1 by Olefogey

Olefogey wrote:
I live in zone 9a and need help with a "weed" that has invaded my St.Augustine lawn.
It is enter twinning in my grass. This is what I have noted in my research.
Root hairs
innate venation
acute tip
serrated edges leaves
opposite side leaves
and flower is purple and very small. It appears to be staving out my grass.
I did use a pre-emergent this spring, but did not take of this.
Can anyone identify?
Olefogey