strcatchar

Strcatchar is a theoretical construct used in the study of string processing and parsing. It denotes an operator that scans a stream of characters and extracts the subsequence that satisfies a given predicate, discarding non-matching characters.

Etymology and usage context: The term combines elements of string, catch, and character. It originated in online programming and theory discussions as a convenient label for a simple filtering mechanism in streaming text tasks. It is commonly used in teaching materials and pseudocode descriptions to illustrate character-level filtering without altering the original input.

Definition and behavior: Given an input sequence x1, x2, ... and a predicate P on characters, strcatchar(P) yields the subsequence { xi : P(xi) is true } in the original order, and it does not modify the input. In streaming contexts, the operator progresses with each incoming character, emitting matches as they arrive. It is typically treated as a filter that can be composed with other processing steps such as mapping or reducing.

Implementation notes: In pseudocode, a simple realization is: for each character c in the input, if P(c) is true, emit c. The approach is memory-efficient when the output size is small relative to the input and is well-suited to pipelines and functional programming layouts. It does not retain discarded characters and does not backtrack to recover them.

Applications and limitations: Strcatchar provides a compact abstraction for tasks like digit extraction or alphabetic filtering in text streams. It is language-agnostic and useful in teaching streaming parsers and data-cleaning workflows. A limitation is that, as a single-pass filter, it cannot recover earlier discarded characters, which can constrain certain parsing strategies.

See also: Filtering, Stream processing, Character encoding, Finite automata, Parser combinators.