How to use sed to replace two instances of the same digits separated by a slash with one instance of those digits

sed 's,\([a-z]\)1\.gif$,\1.gif,g'

or, if you want to allow any non-digit before the 1

sed 's,\([^0-9]\)1\.gif$,\1.gif,g'

The backslash-parenthesis construct delimits a capture group, which the FreeBSD man page calls a “bracket expression” (despite the use of parentheses — square brackets mean something else). Note that sed uses basic regular expressions (BRE), not extended regular expressions (ERE); the man page describes ERE, and the last paragraph explains the difference between BRE syntax and ERE syntax. I find the POSIX specification more readable than the BSD man page here; it calls capture groups back-reference expressions. The GNU sed manual is more readable than either; just avoid the features described as GNU extensions.

Given a capture group (a.k.a. back-reference expression), you can use backslash+digit in the replacement text to mean “the text matched by the corresponding capture group”. For example, \1 in the replacement text is replaced by the text matched by the first capture group in the regular expression. Here there's a single capture group, which captures the letter before 1.gif.

I changed 1.gif to 1\.gif to match the dot literally, and added a trailing $ to match only at the end of the line.

To give another example of capture groups, if you wanted to operate on arbitrary extensions, you could use something like

sed 's,\([^0-9]\)1\(\.[^./]*\)$,\1\2,g'

Escaping the / only is very difficult to do with sed because for instance it would have to be escaped in:

Foo/bar
Foo[XY]/
Foo\[/x\]
Foo\\/bar

But not in

Foo [/x]bar
Foo [^]/x]bar
Foo [x[:blank:]/y]
Foo\/bar

It may be easier to use awk instead

repl() {
  PATTERN=$1 REPL=$2 awk '
    {gsub(ENVIRON["PATTERN"], ENVIRON["REPL"]); print}'
}

However note that awk's regexps are extended regular expressions (as opposed to the basic ones in sed), and while it understand & in the replacement part to mean the matched portion, it does not support sed's \1. Except with busybox awk, it doesn't support back references in the pattern either.

Here you could stick with your approach but document the fact that / needs to be escaped. You'll need to document which are the regexp operators anyway (as the user may need to escape them), that newline can't be matched, and that newline must be escaped in the replacement and the special behaviour of & and backslash there.