Compiler Phases: Which Analyzer Checks Every Character?

In a compiler, the phase that checks every character of the source text is the lexical analyzer (also called the lexer or scanner). It reads the input stream character-by-character and groups characters into tokens (like keywords, identifiers, numbers, operators) using patterns described by the language’s lexical grammar. This behavior is consistent with standard compiler design: after tokenization, the syntax analyzer works at the token level to verify the program’s grammar, while the code generator/optimizer occur later and do not scan raw characters.

Therefore, the correct option is: (i) the lexical analyzer.

Key learning terms: lexical analyzer token syntax analyzer code generator code optimizer

Why the lexical analyzer matches the question

A lexer’s job is to transform a character stream into tokens—so it must examine the input at the character level to recognize token boundaries (e.g., distinguishing while from an identifier while1, or recognizing == as one operator instead of two = characters). After this, the syntax analyzer performs grammatical checks based on the token stream.

Meanwhile:

• syntax analyzer operates on token sequences and ensures the program matches grammar rules (e.g., via parsing tables or recursive descent).
• code generator produces target code from intermediate representation.
• code optimizer rewrites code to improve performance/size.