1. Describe the types of strings that conform to the following regular expressions and construct an example that is matched by the regular expression.

  1. [0-9]+\\$

    This expression is looking for any number of digits preceding a backslash. With the backslash being the end of the string. e.g. 1234\

  2. \\b[a-z]{1,4}\\b

    This expression is looking for a string with a backslash followed by a “b” then one to four letters with another “b” following. The last “b” doesn’t have to be the last element in the string. e.g. \boys\being

  3. .*?\.txt$

    This expression is validating a string with any alphanemeric chracters followed by a backslash then by any character then txt. With txt being the end of the string. e.g.“hello\stxt”

  4. \\d{2}/\\d{2}/\\d{4}

    The expression is validating a backslash followed by two d’s preceding a foward slash followed by a backslash and two d’s followed by another forwardslash and four d’s. e.g.“\dd/\dd/\dddd”

  5. <(.+?)>.+?</\\1>

    This expression is looking for a string that begins with character/s surrounded by a less-than and more than sign preceding that can be any character/word/s which is followed by a less-than sign, a backslash then the number one followed by the more-than sign. e.g. “<hey1234>test</\1>”

  1. Rewrite the expression [0-9]+\\$ in a way that all elements are altered but the expression performs the same task. **“^\d*[\]$“**
#  "^\\d*[\\]$"
pat <- "^\\d*[\\\\]"
st <- "1234\\"
str_match(st,pat)
##      [,1]    
## [1,] "1234\\"

3 Consider the mail address chunkylover53[at]aol[dot]com

  1. Transforming the email address to a standard mail format.
# step 1 find and replace [at] with @
r <- "chunkylover53[at]aol[dot]com"
pat1 <- "\\[at\\]"
r <- str_replace(r,pat1,"@")

r
## [1] "chunkylover53@aol[dot]com"
# step2 find and replace [dot] with .
pat1 <- "\\[dot\\]"
r <- str_replace(r,pat1,".")
r
## [1] "chunkylover53@aol.com"
  1. Using [:digit:] as the regular expression pattern would fail because it would extract the first number it encounters. To fix this we append “+”which will allow us to extract all the digits in the email address.

Before Correction

pat1 <- "[:digit:]"
str_extract(r,pat1)
## [1] "5"

After Correction

pat1 <- "[:digit:]+"
str_extract(r,pat1)
## [1] "53"
  1. Using “\\D” as the regular expression to extract the numbers in the email address would fail because it would identify the first character in the string that is not a number.The correct expression would be “\\d+”.

Before Correction

pat1 <- "\\D"
str_extract(r,pat1)
## [1] "c"

After Correction

pat1 <- "\\d+"
str_extract(r,pat1)
## [1] "53"