How can I replace the last match with a string

Question

Given a regular expression and a replacement string, how can I use awk to implement the following:

• if a line contains no match, does nothing;
• if a line contains more than one matches, replace the last match with the replacement string, in the sense of the last match which would be substituted by gsub();
• if a line contains exactly one match, replace the match with the replacement string.

For example, see " unexpected character '\'" in gawk

Thanks.

Answer 1

Not using gsub() but use your Pattern as the field separator for FS & OFS, then print all the fields except the last two with the OFS intact; next print the second last field followed by the "REPLACEMENT-STRING" and then the last field.

awk 'BEGIN{ FS=OFS="pat" }
      NF>1{ for(i=1; i<NF-1; i++) printf "%s", $i OFS;
            printf "%s\n", $i "REPLACEMENT-STRING" $NF; next
}1' infile

---

Sample input:

        pat 1 pat2 patt 3 pat 4
 pat
patt pat
pa t

Output:

        pat 1 pat2 patt 3 REPLACEMENT-STRING 4
 REPLACEMENT-STRING
patt REPLACEMENT-STRING
pa t