例子：
dis ustrregexra("This is a cat", " is", "X")
dis ustrregexra("This is a cat", "(?<= )is", "X") // 左边是空格的is换为X
dis ustrregexra("This is an island", "(?<= )is", "X")
dis ustrregexra("This is an island","(?<= )is(?= )", "X") // 左边右边都是空格的is换为X
dis ustrregexra("This is a cat", "(?<! )is", "at") // 左边不是空格的is换为X
dis ustrregexra("Hello Wor ld", " (?=[a-z])", "") // 删掉右边为小写字母的空格

clear all
input str100 地址
"合肥市庐阳区濉溪路269号"
"繁昌县繁阳镇迎春西路台客隆超市门面"
"合肥市水阳江路菱水苑北1幢104室"
"安庆市潜山县痘姆乡街道中心"
"东至县大渡口镇南大街安徽长江大市场综合楼1018室"
end
list

gen city1 = ustrregexs(0) if ustrregexm(地址,".+?市") // 提取地址时，“市”常被“超市”“市场”影响，应采取以下方式：

gen city2 = ustrregexs(0) if ustrregexm(地址,".+?(?<!超)市(?!场)")
gen city3 = ustrregexs(0) if ustrregexm(地址,".+(?<!超)市(?!场)")
gen city4 = ustrregexs(0) if ustrregexm(地址,".*(?<!超)市(?!场)")
gen city5 = ustrregexs(0) if ustrregexm(地址,".*?(?<!超)市(?!场)")

list city2-city5