[SOLVED] Using sed to substitute mathematical expressions

Issue

I am struggling to use sed to replace the mathematical expressions. For even though the regular expression \( \- xi\*\*2 \+ 2\*xi \- 1\) rel="nofollow noreferrer">matches the string "( - xi**2 + 2*xi - 1)" sed does not perform the substitution:

echo "( - xi**2 + 2*xi - 1)" | sed -e 's/\( \- xi\*\*2 \+ 2\*xi \- 1\)/k1/g'

Please advise.

Solution

You selected the PCRE2 option at regex101.com, while sed only supports POSIX regex flavor.

Here, you are using a POSIX BRE flavor, so the expression will look like

#!/bin/bash
s="( - xi**2 + 2*xi - 1)"
sed 's/( - xi\*\*2 + 2\*xi - 1)/k1/g' <<< "$s"

See the online demo. In POSIX BRE, this expression means:

• ( - a ( char (when not escaped, matches a ( char)
• - xi - a fixed string
• \*\* - a ** string (* is a quantifier in POSIX BRE that means zero or more)
• 2 + 2 - a fixed 2 + 2 string as + is a mere + char in POSIX BRE
• \* - a * char
• xi - 1) - a literal xi - 1) fixed string, ) in POSIX BRE matches a literal ) char.

If you plan to use POSIX ERE, you will need to escape (, ) and + in your regex:

sed -E 's/\( - xi\*\*2 \+ 2\*xi - 1\)/k1/g' 

Note the difference between -e (that says that the next thing is the script to the commands to be executed) and -E (that means the regex flavor is POSIX ERE).

Answered By - Wiktor Stribiżew

Answer Checked By - Dawn Plyler (WPSolving Volunteer)