Strings and text processing

Strings hold text. Delphi's string functions let you validate, search and transform input — essential for exam questions and real programs.

• `Length(s)` counts characters; `Copy(s, start, n)` extracts a substring; `Pos(sub, s)` finds a substring's position (0 if absent).
• `UpperCase`/`LowerCase` normalise case; `Trim` removes surrounding spaces.
• Access one character with `s[i]`; loop `for i := 1 to Length(s)` to examine every character.
• Test a character with sets: `if s[i] in ['A'..'Z'] then …`.

• String — a sequence of characters.
• Substring — part of a string extracted with Copy.
• Concatenation — joining strings with `+`.