A bank's net interest margin is the difference between its average yield on loans and investments and the average cost for deposits and borrowings. Most regional banks are seeing net interest margins narrow, because they have already seen most of savings on funding costs, as he Federal Reserve has kept its short-term federal funds rate in a target range of between zero and 0.25% since late 2008, while the central bank last month announced it was increasing its purchases of long-term mortgage-backed securities by $40 billion a month, in an effort to keep long-term rates at historically low levels. Jefferies analyst Ken Usdin predicted before the market open that Regions Financial's net interest margin decline would "be seen as disappointing given that the company had guided to stable NIM and flattish net interest income for the back half of the year." The company's net interest declined significantly, to $817 million in the third quarter, from $838 million in the second quarter, and $850 million, in last year's third quarter. Guggenheim analyst Marty Mosby points out that Regions did see its net interest margin increase from 3.04% in the third quarter of2011, and also says that "their margin compressed because last quarter was higher than it should have been," because of recoveries on problem loans. Mosby adds that "earnings from operations increased by 10% from last quarter, which reflects over 80 cents in annualized earnings power." The analyst sees a buying opportunity for investors right now, because "tangible book value that grew 5% and now exceeds seven dollars," and the company's "earnings power can easily justify an $8 stock price." Shares of Regions Financial have now returned 53% year-to-date, following a 38% decline during 2011. The shares for 0.9 times their reported Sept. 30 tangible book value of $7.02, and for eight times the consensus 2013 EPS estimate of 80 cents. RF data by YCharts Interested in more on Regions Financial? See TheStreet Ratings' report card for this stock.