g代表全局，如替换全局（全部）：
IRB中：

>> "hello".sub('l', '*')
=> "he*lo"
>> "hello".gsub('l', '*')
=> "he**o"

不同之处在于sub只替换指定模式的第一个示例，而gsub对所有示例都进行替换（即全局替换）。

value = "abc abc"
puts value                                # abc abc
# Sub replaces just the first instance.
value = value.sub("abc", "---")
puts value                                # --- abc
# Gsub replaces all instances.
value = value.gsub("abc", "---")
puts value                                # --- ---

sub和gsub分别执行第一个和所有匹配的替换。

sub(pattern, replacement, x, ignore.case = FALSE, perl = FALSE,
    fixed = FALSE, useBytes = FALSE)

gsub(pattern, replacement, x, ignore.case = FALSE, perl = FALSE,
     fixed = FALSE, useBytes = FALSE)

sub("4", "8", "An Introduction to R Software Course will be of 4 weeks duration" )  
##"An Introduction to R Software Course will be of 8 weeks duration"

gsub("4", "8", "An Introduction to R Software Course will be of 4 weeks duration" )
##"An Introduction to R Software Course will be of 8 weeks duration"