Using placeholders/variables in a sed command - bash

Question

I want to store a specific part of a matched result as a variable to be used for replacement later. I would like to keep this in a one liner instead of finding the variable I need before hand.
when configuring apache, and use mod_rewrite, you can specificy specific parts of patterns to be used as variables,like this:
RewriteRule ^www.example.com/page/(.*)$ http://www.example.com/page.php?page=$1 [R=301,L]
the part of the pattern match that's contained inside the parenthesis is stored as $1 for use later. So if the url was www.example.com/page/home, it would be replaced with www.example.com/page.php?page=home. So the "home" part of the match was saved in $1 because it was the part of the pattern inside the parenthesis.
I want something like this functionality with a sed command, I need to automatically replace many strings in a SQL dump file, to add drop table if exist commands before each create table, but I need to know the table name to do this, so if the dump file contains something like:
...
CREATE TABLE `orders`
...
I need to run something like:
cat dump.sql | sed "s/CREATE TABLE `(.*)`/DROP TABLE IF EXISTS $1\N CREATE TABLE `$1`/g"
to get the result of:
...
DROP TABLE IF EXISTS `orders`
CREATE TABLE `orders`
...
I'm using the mod_rewrite syntax in the sed command as a logical example of what I'm trying to do.
Any suggestions?

Answer 1

sed '/CREATE TABLE \([^ ]*\)/ s//DROP TABLE IF EXISTS \1; &/'
Find a CREATE TABLE statement and capture the table name. Replace it with 'DROP TABLE IF EXISTS' and the table name, plus a semi-colon to terminate the statement, and a copy of what was matched to preserve the CREATE TABLE statement.
This is classic sed notation. Since you're using bash, there's a chance you're using GNU sed and will need to add --posix to use that notation, or you'll need to fettle the script to use GNU's non-standard sed regexes. I've also not attempted to insert a newline into the output. You can do that with GNU sed if it is important enough to you.
The key points are the parentheses (classically needing to be escaped with a backslash) are the capture mechanism, and backslash-number is the replacement mechanism.

Answer 2

sed -r "s/CREATE TABLE (\`.*\`)/DROP TABLE IF EXISTS \1\n &/g" dump.sql
test:
kent$ cat t.txt
CREATE TABLE `orders`
...
CREATE TABLE `foo`
...
...
CREATE TABLE `bar`
...
kent$ sed -r "s/CREATE TABLE (\`.*\`)/DROP TABLE IF EXISTS \1\n &/g" t.txt
DROP TABLE IF EXISTS `orders`
CREATE TABLE `orders`
...
DROP TABLE IF EXISTS `foo`
CREATE TABLE `foo`
...
...
DROP TABLE IF EXISTS `bar`
CREATE TABLE `bar`
...

Answer 3

This is called a "back reference". And sed will start numbering the things between the parenthesis. \(...\)
Note the use of backslash as an escape character above.
Ref: https://www.gnu.org/software/sed/manual/html_node/Regular-Expressions.html

Answer 4

Here is an answer to the OP's question via title of this thread:
As #bhoom-suktitipat states: This is called a "back reference"...
SUMMARY:
Placeholder variables, or back reference lookup is achieved, on the replace side, by using backslash followed by a digit, starting at 1, like so: \1
BACKGROUND: I'm here because I've been taking a bunch of code and revamping it. In the process I want to pre-eslint it to find all the double quotes around JSON keys and strip them. ESLint's Quote Props Rule can be used, with --fix, to do the exact opposite of what I need todo.
MISSION OUTLINE:
Turn
{ "foo": "bar" }
into { foo: "bar" }, a.k.a. using-placeholders-variables-in-a-sed-command
Find the equivalent of the "foo" (key) in the second matching group: ("{1})(\w+)(":{1}).
Use that matching group as a placeholder to render foo:, instead of "foo":
Write changes to file.
Write to different file:
sed -r 's/("{1})(\w+)(":{1})/\2:/g' in.js > out.js
Write to same file:
sed -ri 's/("{1})(\w+)(":{1})/\2:/g' in-and-out.js
SED flags used:
-E, -r, --regexp-extended
use extended regular expressions in the script
(for portability use POSIX -E).
-i[SUFFIX], --in-place[=SUFFIX]
edit files in place (makes backup if SUFFIX supplied)