Swap text before and after "=" in properties file

Question

Windows 10, Emacs 25.1

Suppose I have properties file:

1=10

2=BS.30

3=BS.20A

Before "=" are numbers, but after "=" has any text(without spaces)

I want to swap text before and after "=".

So the result must be:

10=1

BS.30=2

BS.20A=3

Is it possible?

Answer 1

You can use the command query-replace-regexp which is bound to the key sequence M-%.

Use ^\([^=]*\)=\(.*\)$ as regular expression and \2=\1 as replacement text.

• the leading ^ stands for the empty string at the beginning of line (the match is anchored at the beginning of line)
• the escaped parentheses \(\) delimit groups which you can refer to in the replacement string
• [^=]* means any number of characters that are not the equal sign
• = just stands for itself
• .* are any characters
• $ at the end of the regular expression matches the empty string at the end of line (it anchors the regexp there)