Imagine I have multiple character strings in a list like this:
[[1]]
[1] "1-FA-1-I2-1-I2-1-I2-1-EX-1-I2-1-I3-1-FA-1-"
[2] "-1-I2-1-TR-1-"
[3] "-1-I2-1-FA-1-I3-1-"
[4] "-1-FA-1-FA-1-NR-1-I3-1-I2-1-TR-1-"
[5] "-1-I2-1-"
[6] "-1-I2-1-FA-1-I2-1-"
[7] "-1-I3-1-FA-1-QU-1-"
[8] "-1-I2-1-I2-1-I2-1-NR-1-I2-1-I2-1-NR-1-"
[9] "-1-I2-1-"
[10] "-1-NR-1-I3-1-QU-1-I2-1-I3-1-QU-1-NR-1-I2-1-"
[11] "-1-NR-1-QU-1-QU-1-I2-1-"
I want to use a regex to detect the particular strings where a certain substring precedes another substring, but not necessarily directly preceding the other substring.
For example, let's say that we are looking for FA preceding EX. This would need to match 1 in the list. Even though FA has -1-I2-1-I2-1-I2-1- between itself and EX, the FA still occurs before the EX, hence a match is expected.
How can a generic regex be defined that identifies strings where certain substrings appear before another substring in this manner?
You may use grep.
x <- c("1-FA-1-I2-1-I2-1-I2-1-EX-1-I2-1-I3-1-FA-1-" ,"-1-I2-1-TR-1-")
grepl("FA.*EX", x)
#[1] TRUE FALSE
grep("FA.*EX", x)
#[1] 1