How to include variables values into regular expressions in R - regex

Question

I have 5 files which contain metabolites (details of different bacteria models). I'm writing a function to append a specified number of files. File names look like the following.
[1] "01_iAPECO1_1312_metabolites.csv" "02_iB21_1397_metabolites.csv"
[3] "03_iBWG_1329_metabolites.csv" "04_ic_1306_metabolites.csv"
[5] "05_iE2348C_1286_metabolites.csv"
Below is my function.
strat = 3 # defines the starting position of the range
end = 5 # defines the ending position of the range
type = "metabolites" # two types of files - for metabolites and reactions
files <- NULL
if (type == "metabolites"){
files <- list.files(pattern = "*metabolites\\.csv$")
}else if(type == "reactions"){
files <- list.files(pattern = "*reactions\\.csv$")
}
#reading each file within the range and append them to create one file
for (i in start:end){
temp_df <- data.frame(ModelName = character(), Object = character(),stringsAsFactors = F)
#reading the current file
temp = rbind(one,temp_df)
}
#writing the appended file
write.csv(temp,"appended.csv",row.names = F,quote = F)
temp_df <- NULL
For example, if I specify the start=3 and end = 5, the code is supposed to read files 03, 04 and 05 and append them. Note: the two integers at the beginning of the file names are used to get the file referenced by the range. I'm unable to select the required file within the for loop using a regular expression. When I specify the number it picks up but I'm looking for a generalized version with i in it.
currentFile = grep("01.+",files)
Any help is appreciated.

Answer 1

For the test data shown below this returns a vector containing the file names of the files that start with 02, 03, 04 and 05 and end with "reactions.csv"
# create some test files
for(i in 1:5) cat(file = sprintf("%02djunkreactions[.]csv", i))
# test input
start <- 2
end <- 5
type <- "reactions"
list.files(pattern = paste(sprintf("^%02d.*%s[.]csv$", start:end, type), collapse = "|"))
giving:
[1] "02junkreactions.csv" "03junkreactions.csv" "04junkreactions.csv"
[4] "05junkreactions.csv"
Note: If start and end are both always one digit then a simplification is possible:
list.files(pattern = sprintf("^0[%d-%d].*%s.csv$", start, end, type))

Answer 2

You can do this with a cross-join.
library(dplyr)
library(stringi)
start = 3
end = 5
type = "metabolites"
all_files = data_frame(file = list.files() )
desired_files = data_frame(
number = start:end,
regex = sprintf("^%02.f.*%s", number, file_type) )
all_files %>%
merge(desired_files) %>%
filter(stri_detect_regex(file, regex)) %>%
group_by(number) %>%
do(read.csv(.$file) ) %>%
write.csv("appended.csv", row.names = F, quote = F)

Answer 3

Are you looking for something like this?
files <- c("01_iAPECO1_1312_metabolites.csv", "02_iB21_1397_metabolites.csv","03_iBWG_1329_metabolites.csv", "04_ic_1306_metabolites.csv","05_iE2348C_1286_metabolites.csv")
for(i in 2:4) print(grep(sprintf("^(%02d){1}_",i),files,value=T))