http://maven.apache.org/components/enforcer/enforcer-rules/versionRanges.html

Version Range Specification

The RequireMavenVersion and RequireJavaVersion rules use the standard Maven version range syntax{.externalLink} with one minor change for ease of use (denoted with *):

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  Meaning
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  x >= 1.0 * The default Maven meaning for 1.0 is everything (,) but with 1.0 recommended. Obviously this doesn't work for enforcing versions here, so it has been redefined as a minimum version.
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  x <= 1.0
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  x < 1.0
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  x == 1.0
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  x >= 1.0
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  x > 1.0
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  1.0 < x < 2.0
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  1.0 <= x <= 2.0
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  x <= 1.0 or x >= 1.2. Multiple sets are comma-separated
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  x != 1.1
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