How deeply does The Well-Grounded Rubyist, Fourth Edition cover lookbehind assertion?

Explains positive and negative syntax plus zero-width behavior, without detailing engine implementation.

Lookbehind assertion

As explained in The Well-Grounded Rubyist, Fourth Edition

A lookbehind assertion is a regular-expression assertion that checks text immediately preceding the current match position without consuming that text.

A lookbehind assertion constrains a match by checking the text immediately before the current position. It is zero-width: the preceding text must satisfy the assertion, but it is neither matched nor consumed. section 11.5.3

Require or reject preceding text

Use /(?<=David )BLACK/ to match BLACK only when it follows “David ”. Use /(?<!David )BLACK/ for the opposite condition: match BLACK only when it does not follow “David”.

Distinguish lookbehind from nearby regular-expression constructs:

Lookahead assertions inspect following text without consuming it; lookbehind assertions perform the equivalent check on preceding text. section 11.5.3
Non-capturing groups consume matched characters but omit them from numbered captures; lookbehind assertions consume nothing. section 11.5.3

How The Well-Grounded Rubyist, Fourth Edition covers it

Cover of The Well-Grounded Rubyist, Fourth Edition

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Related concepts

assertionkind of · A regular-expression assertion tests a condition at a position in the input without consuming characters as part of the match.
assertionkind of · A software assertion is an executable check of an expected condition or property that signals failure when the check does not hold.
regular expressionused by · A formal pattern notation that describes sets of strings and enables text to be matched, searched, extracted, or transformed.
alternationused by · Regular-expression alternation is a construct that matches one of several alternative subpatterns, typically separated by a vertical bar.
Pythoncontrasts with · Python is a high-level, general-purpose, dynamically typed programming language known for readable syntax and automatic memory management.

This explainer was generated by Manning's knowledge graph from the text of The Well-Grounded Rubyist, Fourth Edition. The section links point to the parts of the book it draws on.