Cac ky thuat regex thuong gap

De bai

Ta co vector x_1

# code chunk
# dau comment la dau # khi trong code chunk
x_1 <- c("ab*c", "bac*", "cab", "aab*b", "a", "b*", "c", "ccbb", " ", "c a b", "bbbca")
x_1

##  [1] "ab*c"  "bac*"  "cab"   "aab*b" "a"     "b*"    "c"     "ccbb"  " "    
## [10] "c a b" "bbbca"