R语言多行文本列替换行首字符：为何^o正则表达式不生效？

问题背景 #

我需要把下面文本里所有行首的o 替换成• ：

• Direct the Department’s technical
• Perform supervisory and managerial responsibilities as leader of the program
o Set direction to ensure goals and objectives
o Select management and other key personnel
o Collaborate with executive colleagues to develop and execute corporate initiatives and department strategy
o Oversee the preparation and execution of department’s Annual Financial Plan and budget
o Manage merit pay
• Perform other duties as assigned

试了三段R代码：

test<- sub(test, pattern = "o ", replacement = "• ")  # 没用（只替换第一个匹配项，还没限定行首）
test<- gsub(test, pattern = "^o ", replacement = "• ")  # 也没用
test<- gsub(test, pattern = "o ", replacement = "• ") # 能替换但会把"to"改成"t•"

问题根源 #

R里的正则默认规则是：^只匹配整个字符串的开头，不是每一行的开头。你的文本是一个包含多行的字符串，那些行首的o 都是换行之后的行开头，不是整个字符串的起始位置，所以^o 根本碰不到这些位置，自然替换失败。

解决办法 #

要让^匹配每一行的开头，得开多行模式，在R里只要在正则开头加(?m)就行：

test <- gsub(test, pattern = "(?m)^o ", replacement = "• ")

这样就只会替换每行开头的o ，不会误伤文本中间的"o"了。

内容的提问来源于stack exchange，提问作者user3482393