//m causes ^ and $ to match at newlines in the middle of your string, while //s causes . to match newlines.

You're comparing using /m to not using /m, and using /s to not using /s. You're not comparing /m to /s.

Your river example compares the location of the rivers, but there's no common aspect of /m and /s to compare.

In reply to Re^3: example of 'm / / m' related example and compare to 'm / / s' by ikegami
in thread PERL regex modifiers for m// by rockstar99

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