$str='\add[sometext]{\begin{equation}\label{eqn:3} f_{1} = \begin{cases}} \beta_{1} + \beta_{2}f_{2} & f_{2}\leq \gamma\\ \beta_{1} + \beta_{2}\gamma + \beta_{4}(f_{2}-\gamma) & f_{2} >\gamma \end{cases}sdsdssd, \end{equation}} it may have some extra code here with {dfdfd} \end{equation}}and some text with so many curly braces }}'
\begin{equation}\label{eqn:3} f_{1} = \begin{cases} \beta_{1} + \beta_{2}f_{2} & f_{2}\leq \gamma\\ \beta_{1} + \beta_{2}\gamma + \beta_{4}(f_{2}-\gamma) & f_{2} >\gamma \end{cases}sdsdssd, \end{equation}
$str=preg_replace('/\\\\add\s*\[\s*\w*\]\s*{(.*?)}/s,$1,$match)
$str = preg_replace('/\\\\add\s*\[.*]\s*{(.*?)\\\\end{(.[^\s]*?)}}/s', "$1\\end{\$2}", $str);
var
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