Improved coerce regex and added coerce to README.

- updated README with new CLI usage and a section for the coerce function
- minimized how much was consumed by the coerce regex
- added all coercion examples from the README to the coerce tests

Author: joeledwards

README.md

@@ -20,6 +20,8 @@ semver.clean('  =v1.2.3   ') // '1.2.3'
 semver.satisfies('1.2.3', '1.x || >=2.5.0 || 5.0.0 - 7.2.3') // true
 semver.gt('1.2.3', '9.8.7') // false
 semver.lt('1.2.3', '9.8.7') // true
+semver.valid(semver.coerce('v2')) // '2.0.0'
+semver.valid(semver.coerce('42.6.7.9.3-alpha')) // '42.6.7'
 ```
 
 As a command-line utility:
@@ -52,6 +54,10 @@ Options:
 -l --loose
         Interpret versions and ranges loosely
 
+-c --coerce
+        Coerce a string into SemVer if possible
+        (does not imply --loose)
+
 Program exits successfully if any valid version satisfies
 all supplied ranges, and prints all satisfying versions.
 
@@ -364,3 +370,15 @@ satisfy the range.
 
 If you want to know if a version satisfies or does not satisfy a
 range, use the `satisfies(version, range)` function.
+
+### Coercion
+
+* `coerce(version)`: Coerces a string to semver if possible
+
+This aims to provide a very forgiving translation of a non-semver
+string to semver. It looks for the first digit in a string, and
+consumes all remaining characters which satisfy at least a partial semver
+(e.g., `1`, `1.2`, `1.2.3`) up to the max permitted length (256 characters).
+Longer versions are simply truncated (`4.6.3.9.2-alpha2` becomes `4.6.3`).
+All surrounding text is simply ignored (`v3.4 replaces v3.3.1` becomes `3.4.0`).
+Only text which lacks digits will fail coercion (`version one` is not valid).

semver.js

@@ -1306,7 +1306,7 @@ function coerce(version) {
   if (version.length > MAX_LENGTH)
     return null;
 
-  var match = version.match(/([vV]?\d+(?:[.]\d+)*)/)
+  var match = version.match(/([vV]?\d+(?:[.]\d+){0,2})/)
   if (match == null)
     return null
 

test/coerce.js

@@ -14,6 +14,7 @@ test('\ncoerce tests', function(t) {
     function () { return '1.2.3'; },
     '',
     '.',
+    'version one',
     tooLong
   ].forEach(function (input) {
     var msg = 'coerce(' + input + ') should be null'
@@ -60,6 +61,10 @@ test('\ncoerce tests', function(t) {
     ['version1', '1.0.0'],
     ['version1.0', '1.0.0'],
     ['version1.1', '1.1.0'],
+    ['42.6.7.9.3-alpha', '42.6.7'],
+    ['v2', '2.0.0'],
+    ['v3.4 replaces v3.3.1', '3.4.0'],
+    ['4.6.3.9.2-alpha2', '4.6.3'],
     [justRight, '12.1.1']
   ].forEach(function (tuple) {
     var input = tuple[0]