Filter rows where mirror-image delimiters are not paired

May use the pattern (\\[[^\\]]+(\\[|$)|(^|\\])[^\\[]+\\]) in str_detect

library(dplyr)
library(stringr)

df %>%
   filter(str_detect(Utterance, "\\[[^\\]]+(\\[|$)|(^|\\])[^\\[]+\\]"))
  id                                Utterance
1  1                    [but if I came !ho!me
2  3                  =[yeah] I mean [does it
3  4                     bu[t if (.) you know
4  5                                =ye::a:h]
5  6  [that's right] YEAH (laughs)] [ye::a:h]
6  8 [cos] I've [heard very sketchy [stories]
7  9                    oh well] that's great

Here we check for a opening bracket [ followed by one or more characters that are not ] followed by a [ or the end of the string ($) or a similar pattern for the closing bracket

Another possible solution, using purrr::map_dfr.

EXPLANATION

I provide, in what follows, an explanation for my solution, as asked for by @ChrisRuehlemann:

1. With str_extract_all(df$Utterance, "\\[|\\]"), we extract all [ and ] of each utterance as a list and according to the order they appear in the utterance.

2. We iterate all lists created previously for the utterances. However, we have a list of square brackets. So, we need to beforehand collapse the list into a single string of square brackets (str_c(.x, collapse = "")).

3. We compare the string of square brackets of each utterance with a string like the following [][][]... (str_c(rep("[]", length(.x)/2), collapse = "")). If these two strings are not equal, then square brackets are missing!

4. When map_dfr finishes, we end up with a column of TRUE and FALSE, which we can use to filter the original dataframe as wanted.

library(tidyverse)    

str_extract_all(df$Utterance, "\\[|\\]") %>% 
  map_dfr(~ list(OK = str_c(.x, collapse = "") != 
            str_c(rep("[]", length(.x)/2), collapse = ""))) %>% 
  filter(df,.)

#>   id                                Utterance
#> 1  1                    [but if I came !ho!me
#> 2  3                  =[yeah] I mean [does it
#> 3  4                     bu[t if (.) you know
#> 4  5                                =ye::a:h]
#> 5  6  [that's right] YEAH (laughs)] [ye::a:h]
#> 6  8 [cos] I've [heard very sketchy [stories]
#> 7  9                    oh well] that's great