13.2 Regular expressions to find more flexible patterns

Special characters used for pattern recognition:

Character	Description
$	Find pattern at the end of the string
^	Find pattern at the beginning of the string
{n}	The previous pattern should be found exactly n times
{n,m}	The previous pattern should be found between n and m times
+	The previous pattern should be found at least 1 time
*	One or more allowed, but optional
?	One allowed, but optional

Match your own pattern inside []

Pattern	Description
[abc]	matches a, b, or c
1	matches a, b or c at the beginning of the element
^A[abc]+	matches A as the first character of the element, then either a, b or c
^A[abc]*	matches A as the first character of the element, then optionally either a, b or c
^A[abc]{1}_	matches A as the first character of the element, then either a, b or c (one time!) followed by an underscore
[a-z]	matches every character between a and z
[A-Z]	matches every character between A and Z
[0-9]	matches every number between 0 and 9

• Match anything contained between brackets (here either g or t) once:

grep(pattern="[gt]", 
    x=c("genomics", "proteomics", "transcriptomics"), 
    value=TRUE)

• Match anything contained between brackets once AND at the start of the element:

grep(pattern="^[gt]",
        x=c("genomics", "proteomics", "transcriptomics"),
        value=TRUE)

• Create a vector of email addresses:

vec_ad <- c("marie.curie@yahoo.es", "albert.einstein01@hotmail.com", 
    "charles.darwin1809@gmail.com", "rosalind.franklin@aol.it")

• Keep only email addresses finishing with “es”:

grep(pattern="es$",
        x=vec_ad,
        value=TRUE)

---

1. abc↩