src/pkg/path/filepath/path_windows.go

// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package filepath

// IsAbs returns true if the path is absolute.
func IsAbs(path string) (b bool) {
	v := volumeName(path)
	if v == "" {
		return false
	}
	path = path[len(v):]
	if path == "" {
		return false
	}
	return path[0] == '/' || path[0] == '\\'
}

// volumeName return leading volume name.  
// If given "C:\foo\bar", return "C:" on windows.
func volumeName(path string) (v string) {
	if len(path) < 2 {
		return ""
	}
	// with drive letter
	c := path[0]
	if path[1] == ':' &&
		('0' <= c && c <= '9' || 'a' <= c && c <= 'z' ||
			'A' <= c && c <= 'Z') {
		return path[:2]
	}
	return ""
}