re模块的函数-sub()

sub(pattern, repl, string, count=0, flags=0)
替换函数，将正则表达式 pattern 匹配到的全部字符串替换为 repl 指定的字符串,
参数 count 用于指定最大替换次数.

import fileinput,re
str01='[x=2] [y=2] The sum of [x] and [y] is [x+y]'
field_pat = re.compile(r'\[(.+?)\]')
scope={}
def replacement(match):
    code=match.group(1)
    try:
        return str(eval(code,scope))#处理[x],[y],[x+y]
    except SyntaxError:
        exec code in scope #处理[x=2],[y=2].
        return ''

##lines = []
##for line in fileinput.input('test.txt'):
##    lines.append(line)
##    text=''.join(lines)
print(field_pat.sub(replacement,str01))