Regex too complicated

Not a catch per say (IIRC you cannot catch it, especially if Catch Exception did not work) but will solve the problem of regex to complex, use a custom iterator:

Class

public with sharing class Utility_RowIterator implements Iterator<String>, Iterable<String>
{
private String m_Data;
private Integer m_index = 0;
private String m_rowDelimiter = '\n';

public Utility_RowIterator(String fileData)
{
m_Data = fileData;
}
public Utility_RowIterator(String fileData, String rowDelimiter)
{
m_Data = fileData;
m_rowDelimiter = rowDelimiter;
}

public Boolean hasNext()
{
return m_index < m_Data.length() ? true : false;
}
public String next()
{
Integer key = m_Data.indexOf(m_rowDelimiter, m_index);

if (key == -1)
key = m_Data.length();

String row = m_Data.subString(m_index, key);
m_index = key + 1;
return row;
}
public Iterator<String> Iterator()
{
return this;
}
}

Code

Utility_RowIterator r = New Utility_RowIterator(csv,'\n'); //Replace \n with whatever delineates your row

String firstRow;
if(r.hasNext()) firstRow = r.next();