Search and replace a regex in multiple files on UNIX

grep search & replace

egrep -lRZ "\.jpg|\.png|\.gif" . \
    | xargs -0 -l sed -i -e 's/\.jpg\|\.gif\|\.png/.bmp/g'

#egrep: find matching lines using extended regular expressions
#  -l: only list matching filenames
#  -R: search recursively through all given directories
#  -Z: use \0 as record separator
#  "\.jpg|\.png|\.gif": match one of the strings ".jpg", ".gif" or ".png"
#  .: start the search in the current directory
#xargs: execute a command with the stdin as argument
#  -0: use \0 as record separator. This is important to match the -Z of egrep and to avoid being fooled by spaces and newlines in input filenames.
#  -l: use one line per command as parameter (NOTE -d for macs, and you must provide a number following
#sed: the stream editor
#  -i: replace the input file with the output without making a backup
#  -e: use the following argument as expression
#  's/\.jpg\|\.gif\|\.png/.bmp/g': replace all occurrences of the strings ".jpg", ".gif" or ".png" with ".bmp"

or with perl:

perl -i.bak -pe 's/.jpg|.png|.gif/.jpg/