Using grep on files with no newlines

unix / linux / regex

I had a big, automatically-generated HTML file without newlines and wanted to see all occurrences of a certain image name. Using the command grep my_image.png just outputs the whole file - not useful! While writing the question to the unix section of StackExchange, I found a way to do it with grep:

grep -oE ".{30}my_image.{30}"

This command yielded the desired result: Each occurrence on a separate line, with 30 characters before and after.

Explanation of the command line

  • The -o option changes the default grep behavior (print matched line) to printing only the matched string.
  • The -E activates the matching of regular expressions, which I use in the matching expression.
  • .{30} is a regular expression meaning “any 30 characters”, so I can see the surrounding context of the match. To avoid missing matches that are not in the first/last 30 characters of the file I could use the expression .{0,30} instead, but that will slow down the search, because the regular expression engine will try all combinations between 0 and 30 characters.