By default, searching does not use regular expressions, and is case-insensitive. You can turn on regular expressions by using the option search = list(regex = TRUE), and perform case-sensitive searching by list(search = caseInsensitive = FALSE) (the two options are orthogonal).

library(DT)
mtcars2 = mtcars[, c(1:5, 9)]
mtcars2$am = factor(mtcars$am, c(0, 1), c('automatic', 'manual'))
# search for Ma or Me
datatable(
  mtcars2, colnames = c('model' = 1),
  filter = list(position = 'top', clear = FALSE),
  options = list(
    search = list(regex = TRUE, caseInsensitive = FALSE, search = 'M[ae]'),
    pageLength = 5
  )
)

Please note the interpretation of regular expressions depends on whether you use client-side or server-side processing. For client-side processing, the search string is treated as JavaScript regular expressions. For server-side processing, it is up to the server-side script. In DT, the server-side processing function is implemented in R, so the search string is treated as R regular expressions.

In the above example, we specified an initial search string M[ae] (meaning Ma or Me) in the global search box. We can also specify initial search strings for individual columns via the searchCols option when column filters are enabled. The searchCols option is a list of the same length as the number of columns of the table. Each element in the list is either NULL or a list of the form list(search = 'keyword'). In the example below, we search for er in the first column, filter the second column using [15, 25], and filter the last column by "automatic".

datatable(
  mtcars2, colnames = c('model' = 1),
  filter = list(position = 'top', clear = FALSE),
  options = list(pageLength = 5, searchCols = list(
    list(search = 'er'),
    list(search = '15 ... 25'),
    NULL, NULL, NULL, NULL,
    list(search = '["automatic"]')
  ))
)