$re = '/(?<!^)((?<=[a-z])[A-Z0-9]|[A-Z](?=[a-z]))/m';
$str = 'Situacao",
PES_TIPO_PESSOA",
CedenteCnpjCpf",
TIT_CEDENTE_ENT_CODIGO",
CedenteNome",
Cnae",
SecaoCNAEDescricao",
NotaPdd",
SAC_TIPO_PESSOA",
SacadoCnpjCpf",
SacadoNome",
IdTituloVortx",
TipoAtivo",
DataEmissao",
DataAquisicao",
DataVencimento",
NumeroBoleto",
NumeroTitulo",
CampoChave",
ValorAquisicao",
ValorNominal",
ValorPresente",
PDDNota",
PDDVencido",
PagamentoParcial",
Coobricacao",
DataGeracao",
PDDTotal",
CampoAdicional1",
CampoAdicional2",
CampoAdicional3",
CampoAdicional4",
CampoAdicional5,
Campo22Adicional5
camelCase';
$subst = "_\1";
$result = preg_replace($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