How Does the Order of Regular Expression Operators Affect Matching in Alternation Groups (..|. .. .|..)?

Order of Regular Expression Operators in (..|. .. .|..)

The order of regular expression operators in (..|. .. .|..) follows a left-to-right precedence. This means that the first option matched will be the result, and the other options will not be evaluated.

This behavior is typical of Non-Deterministic Finite Automata (NFA) regexes. For a detailed explanation, refer to the "Alternation" page at regular-expressions.info.

It's important to note that the RegexOptions.RightToLeft flag only affects the direction the regex engine examines the input string, not the order of pattern processing.

For example, with the regex (aaa|bb|a), matching "bbac" with Regex.Match returns "bb" because the "a" alternative appears after "bb." However, Regex.Matches will return both "bb" and "a."

Additionally, within non-anchored alternative groups, the order of alternatives is significant. For instance, (a|aa|aaa) matching "abbccaa" will match every "a" character. But once word boundaries are added, the order becomes irrelevant, as demonstrated in this regex demo.

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