How Does Operator Order Affect Matching in Regular Expressions?

Operator Order in Regular Expressions: (..|.. ... ..|..)

In regular expressions, the order of precedence for expressions in (..|.. ... ..|..) is left to right. The first alternative that matches "wins," and subsequent alternatives are not evaluated. This behavior is characteristic of nondeterministic finite automata (NFAs), which are commonly used in regex engines.

Left-to-Right Evaluation

For example, the expression (aaa|bb|a) will match "bb" in the string "bbac" because "bb" appears before "a" in the regex pattern. If you used Regex.Matches instead, both "bb" and "a" would be matched.

Order of Alternatives

Within a non-anchored alternative group, the order of alternatives matters. For instance, the expression (a|aa|aaa) will match each "a" in the string "abbccaa."

However, when using word boundaries to anchor the expression, the order of alternatives becomes irrelevant. For example, (.)a(.|$) will match "a" in "abbccaa" regardless of the order of the alternatives ".*" and "$."

Note on the RegexOptions.RightToLeft Flag

It's important to note that the RegexOptions.RightToLeft flag only affects the direction in which the input string is scanned, not the order in which the regex pattern is processed.

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