r doesn't see data frame as characters; can't grep; wrong use of as.character()?

EDIT, A DAY LATER: The answers to this question have shown me that I needed to edit my code quite substantially. So basically the question has now gone away because I'm not grepping in a data-frame any more. The code is now as follows, much cleaner.

I'm leaving the original question here in case my learning process helps anyone, though.

# 1. Find lines containing both "un" and "ɛ̃"
original_lines <- readLines('Test.txt')
lines_with_pattern <- grep('un.*ɛ̃', original_lines, value = TRUE)

# CHANGE PHONES TO FIND AND PHONES TO ADD

# 2. Duplicate the line in which the pattern occurs and change the relevant phoneme
modified_lines <- character()
for (line in lines_with_pattern)
  modified_lines <- c(modified_lines, gsub("ɛ̃", "œ̃", line))

# 3. Combine modified lines with original lines
all_lines <- c(original_lines, modified_lines)

# 4. Sort the lines alphabetically
sorted_lines <- sort(all_lines)

# 5. Print the sorted lines
writeLines(sorted_lines, 'myfile.txt', sep = '\\n')

ORIGINAL QUESTION

I am trying to grep a data frame consisting of rows of two columns with tab separation between the columns, e.g.

              V1            V2
17 nempruntèrent ɑ̃ p ʁ ɛ̃ t ɛ ʁ
18     vemprunté   ɑ̃ p ʁ ɛ̃ t e
19    fempruntée   ɑ̃ p ʁ ɛ̃ t e
20   wempruntées   ɑ̃ p ʁ ɛ̃ t e
21    2empruntés   ɑ̃ p ʁ ɛ̃ t e

(excerpt--the last five rows of the data frame. The first col contains dummy French-like words; the second col contains International Phonetic Alphabet transcriptions.)

test <- read.delim('Test.txt', header=FALSE)
print(test)

produces a printout as above, so it looks as if R 'knows' what's in the data frame.

But then I want to grep for certain strings, so I've tried

# 1. Find lines containing both "un" and "ɛ̃"
lines_with_pattern <- grep("un", test, value = TRUE)
print(lines_with_pattern)

and this doesn't work.

The above grep results in named character(0). This means R is not finding the characters it's looking for, so I've tried

test <- read.delim('Test.txt', header=FALSE)
test <- as.character(test)

I don't think I'm using as.character() right, as that snippet produces e.g.

     V1 V2
 
[17,] NA NA
[18,] NA NA
[19,] NA NA
[20,] NA NA
[21,] NA NA

(again the last five rows of the output)

and therefore print(test) produces

[1] "c(17, 18, 12, 20, 1)"
[2] "c(8, 6, 6, 6, 6)"

(last five figures in the resulting vectors)

and

lines_with_pattern <- grep("un", test, value = TRUE)
# value = TRUE, fixed = FALSE, useBytes = TRUE, invert = FALSE)
print(lines_with_pattern)

produces character(0).

So: I don't understand the vectors that print(test) produces in the example just above--the numbers don't seem to refer to anything corresponding to the data. And, my original question: what do I need to do to be able to grep this data-set?

Sorry for the very long message, and for the noob question, but thanks a lot for any help!