grep using a character vector with multiple patterns

Question

I am trying to use `grep` to test whether a vector of strings are present in an another vector or not, and to output the values that are present (the matching patterns). I have a data frame like this: ``` FirstName Letter Alex A1 Alex A6 Alex A7 Bob A1 Chris A9 Chris A6 ``` I have a vector of strings patterns to be found in the "Letter" columns, for example: `c("A1", "A9", "A6")`. I would like to check whether the any of the strings in the pattern vector is present in the "Letter" column. If they are, I would like the output of unique values. The problem is, I don't know how to use `grep` with multiple patterns. I tried: ``` matches <- unique ( grep("A1| A9 | A6", myfile$Letter, value=TRUE, fixed=TRUE) ) ``` But it gives me 0 matches which is not true, any suggestions?

Accepted Answer

In addition to @Marek's comment about not including `fixed==TRUE`, you also need to not have the spaces in your regular expression. It should be `"A1|A9|A6"`.

You also mention that there are lots of patterns. Assuming that they are in a vector

```
toMatch <- c("A1", "A9", "A6")
```

Then you can create your regular expression directly using `paste` and `collapse = "|"`.

```
matches <- unique (grep(paste(toMatch,collapse="|"), 
                        myfile$Letter, value=TRUE))
```

grep using a character vector with multiple patterns

Solution

Problem