# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/(?<=if \(|elseif )\$\{([^}]*)\}([+-])\$\{([^}]*)\}/\1\2\3/gm;t' <<< "<#if (${val1}-${val2}) + ${f1(${a}+${b}*${c})} > 2000>ABC<#elseif ${n1}+${n2}* ${f1(${n3}-${n4})} < 500>DEF</#if>"
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for SED, please visit: https://www.gnu.org/software/sed/manual/html_node/The-_0022s_0022-Command.html