$re = '/\$[^\$]*\$/';
$str = '$hello$
$
hello
$
.projection $I_{d,i} \\rightarrow M_{d,i} M_{d,0}$, where all the
\\begin{itemize}
\\item $M$ is atomic and finitely generated;
\\item $M$ is cancellative;
\\item $(M, \\le_L)$ and $(M, \\le_R)$ are lattices;
\\item there exists an element $\\Delta \\in M$, called {\\it Garside element}, such that the set
$L(\\Delta)= \\{ x \\in M; x\\le_L \\Delta\\}$ generates $M$ and is equal to $R(\\Delta)= \\{ x\\in M;
x\\le_R \\Delta\\}$.
\\end{itemize}
Using the fact that is atomic and that $L(\\Delta)=
\\{x \\in M; x \\le_L \\Delta\\} M \\pi_L(a) \\neq 1 a \\neq
1 k \\partial_L^k(a)=1 k$ be the
';
$subst = " ";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
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 PHP, please visit: http://php.net/manual/en/ref.pcre.php