Yes and no. An espresso cup has about as much caffeine as a cup of strong coffee. But servings for espresso are much smaller. Which means that the content of caffeine per milliliter are much higher than with a regular brew. Moreover, caffeine is more quickly assimilated when taken in concentrated dosages, such as an espresso cup.
The myth of lower caffeine espresso comes comes from the fact that the darker roast beans used for espresso do have less caffeine than regularly roasted beans as roasting is supposed to break up or sublimate the caffeine in the beans (I have read this quote in research articles, but found no scientific studies supporting it. Anybody out there?).
One more thing that should be considered when comparing caffeine content of espresso is whether the beans are arabica or robusta. Robusta has about twice as much caffeine as arabica therefore a coffee blend starting with a large amount of robusta will have more caffeine regardless of prep method. Many (but not all) supermarket brands of coffee have a fair amount of robusta mixed into the blend to keep production costs low. Some espresso blends have between 4% and 12% robusta. The robusta is uses for a combination of reasons not least of which is flavor and better crema production. Many espresso blends have no robusta at all. For a good espresso blend price is not the reason for adding robusta and a good quality robusta is actually much more expensive than a cheap arabica bean and somewhere on par with a similar relative quality arabica.
Here’s the caffeine content of Drip/Espresso/Brewed Coffee:
Drip 115-175 (7 oz cup) Espresso 100 1 serving (1-2oz) Brewed 80-135 (7 oz cup)
Obviously these numbers are estimates at best.