$re = '/{tex\b([^{}]*({[^{}]*?([^{}]|(?R))*[^{}]*?}[^{}]*)*[^{}]*)}/m';
$str = '# This is currently a test
{nav Home}
The statistical model estimated the probability, {tex f(x) = \\int_{-\\infty}
^\\infty \\hat f(\\xi)\\,e^{2 \\pi \\xi x} \\,d\\xi }, of capturing dolphins on a tow, {tex i}. A year effect, {tex \\lambda_{acb_{j}}} was estimated for each year, {tex j}, allowing for annual variation in the capture event rates that was unrelated to the covariates, {tex x}. The contribution of each covariate, indexed by {tex c}, was governed by a regression coefficient, {tex \\beta_c}, that was estimated by the model. The logit transform of the capture event probability was defined as the sum of the year effect, {tex \\lambda_{lam} {lam} }, and the covariates:
{tex \\lambda_{lam}{lam} }
{tex \\lambda_{lam} {lam} }
{tex \\lambda_{lam}{lam}}
katex {{{
logit(\\pi_i) = \\lambda_{j[i]} + \\sum_c \\beta_c x_{ic}.
}}}
determines the result.
# The following is more tests
katex {{{
f(x) = \\int_{-\\infty}^\\infty
\\hat f(\\xi)\\,e^{2 \\pi i \\xi x}
\\,d\\xi
}}}
katex {{{
\\begin{bmatrix}
a & b \\\\
c & a
\\end{bmatrix}
}}}
katex {{{
\\begin{bmatrix}
a & c \\\\
c & a
\\end{bmatrix}
}}}
';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
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