* Dollar/yen hits highest since August 2009 * BOJ pledges to pump $1.4 trillion into economy * Euro steady after falling to 4-1/2 yr low on Thursday By Sophie Knight TOKYO, April 5 (Reuters) - The yen slid to a 3-1/2 year low against the dollar on Friday, after suffering its biggest one-day tumble since late 2008 on Thursday, when the Bank of Japan surprised markets with a radical campaign of monetary expansion to attack deflation. The dollar rose as high as 97.20 yen on trading platform EBS, a level not seen since August 2009. It last stood at 97.13 yen, up 0.8 percent on the day. The greenback soared 3.6 percent against the yen on Thursday after the BOJ unleashed the intense monetary stimulus, promising to inject about $1.4 trillion into the economy in less than two years, a gamble that sent bond yields to record lows as prices rose on the prospect of massive BOJ bond-buying. Governor Haruhiko Kuroda, chairing his first policy meeting, committed the BOJ to open-ended asset buying and said the monetary base would nearly double to 270 trillion yen ($2.9 trillion) by the end of 2014. "They've committed to expanding the monetary base at quite a significant pace, which has had a very big impact on the market," said Bill Diviney, currency strategist at Barclays Capital in Tokyo. The yen continued to lose ground across the board. Against the euro, it slipped 0.6 percent to a three-week low of 125.61 yen. The common currency jumped 4.3 percent on Thursday, its biggest one-day rise against the yen since November 2008. The euro breezed into the 125-125.50 yen range, but faces some resistance before the 126 level and is some way off from a high of 127.71 yen hit on Feb.6, according to the EBS platform. Analysts says European bonds could benefit as the Japanese yield curve has already flattened on expectations of the BOJ purchasing long-dated Japanese government debt, which could pique domestic investors' hunger for higher yields abroad. The BOJ's new plan means it will buy about 7 trillion yen ($73 billion) of bonds per month, equivalent to about 1.4 percent of gross domestic product. By comparison, the U.S. Federal Reserve is buying $85 billion of bonds per month, about 0.6 percent the size of the economy. "Domestic investors are likely to be encouraged by the BOJ's promise to stay accommodative for quite some time," said Diviney of Barclays, adding that he expected U.S. assets to see the most significant boost from Japanese investors' forays abroad. Domestic retail investors have already begun sniffing out riskier assets. Subscriptions for an emerging markets equities fund launched last week for assets in countries such as Turkey, Indonesia and Thailand reached nearly $1.6 billion, the highest for a single launch since December. Analysts say that the dollar has space to run higher against the yen now that it has regained a foothold above the 96 yen level. Barclays foresees the dollar firming to 103 yen in the coming weeks, while Societe Generale sees that level as a long-term target that should be reached by the first quarter of 2014. " stay bearish yen for now. External forces, namely softer US data and euro area-led deterioration of risk conditions this spring, may well temper the JPY sell-off in Q2," said a Societe Generale report. U.S. DATA Some investors remained cautious in the near-term as they awaited the U.S. nonfarm payrolls report later on Friday, as a disappointing result could keep U.S. bond yields depressed and add to expectations of more bond-buying from the Federal Reserve, which would weigh on the dollar. Data showing weaker-than-expected growth in the U.S. private-sector employment and initial jobless claims at four-month highs last week has led to worries that the labour market is losing momentum. However, some say that market expectations have already been adjusted. "Weak employment data this week means a disappointing payroll report has been priced in. However, that makes a reaction to a positive surprise more likely," said Junya Tanase, executive director of FX research at JPMorgan in Tokyo. Against the dollar, the euro slipped 0.1 percent to 1.2926 after a sharp reversal on Thursday. The common currency moved from a 4-1/2 month low around $1.2745 as investors pared back their bearish bets against the single currency, prompting it to rise 0.7 percent by late U.S. trade. European Central Bank President Mario Draghi said on Thursday the bank stood ready to act if growth continues to languish. He also affirmed his commitment to keeping the euro zone intact and said the Cyprus bailout was not a "template" for future rescues in the currency zone.