Text Processing: Strings, Streams and Regular Expressions

Slicing, parsing, building and validating text with the C++ standard library

Big Idea: Almost every real program eats text and produces text: config files, CSV rows, log lines, user input, network messages. C++ gives you three cooperating tools for this work. std::string stores and edits the characters. std::stringstream turns text into typed values and back again. std::regex describes and matches patterns. Used together they let you split a line, extract a number, and confirm an email looks valid without ever touching a raw char* or a hand-written parser loop.

By the end you will be able to:

search, slice, and rebuild text with std::string member functions,
parse whitespace- and delimiter-separated fields with std::istringstream and build output with std::ostringstream,
write std::regex patterns and choose between regex_match, regex_search and regex_replace,
pull captured groups out of a match and validate real-world input safely.

1. std::string: search, slice, rebuild

You met std::string in the beginner course as "a container of characters". At this level it pays to know it as a genuine sequence with a rich, index-based API from the <string> header. Three operations do most of the day-to-day work: find, substr, and replace / append.

s.find("x") returns the index of the first occurrence, or the special constant std::string::npos if it is not present. Always compare against npos - never against -1.
s.substr(pos, len) returns a new string copied from pos; if you omit len it runs to the end.
s.replace(pos, len, repl), s.insert(pos, txt), s.erase(pos, len) and s += "..." edit in place.
s.size() (same as length()), s.empty(), s[i] for unchecked access, and s.at(i) for bounds-checked access that throws std::out_of_range.

std::string path = "report.final.csv";
std::size_t dot = path.rfind('.');          // search from the RIGHT
std::string ext = (dot == std::string::npos)
                  ? ""
                  : path.substr(dot + 1);   // "csv"

Notice rfind: it finds the last dot, which is exactly what you want for a file extension. The guard against npos is not optional - calling substr with a bad position throws.

Text Processing: Strings, Streams and Regular Expressions

Text Processing: Strings, Streams and Regular Expressions

1. std::string: search, slice, rebuild

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