diff --git a/.golangci.yml b/.golangci.yml
index d0f65ba..f1de1e6 100644
--- a/.golangci.yml
+++ b/.golangci.yml
@@ -34,3 +34,6 @@ linters-settings:
       - standard
       - default
       - localmodule
+  gomoddirectives:
+    replace-allow-list:
+      - github.com/karrick/golf
diff --git a/README.md b/README.md
index 4af0721..243f153 100644
--- a/README.md
+++ b/README.md
@@ -35,9 +35,9 @@ supports the following command-line flags:
 
 ### DNS zone serials
 
-  The `check_zone_serial` plugin can be used to check that the version of a
-  zone served by a DNS is up-to-date compared to the same zone served by
-  another, "reference" DNS. It supports the following command-line flags:
+The `check_zone_serial` plugin can be used to check that the version of a
+zone served by a DNS is up-to-date compared to the same zone served by
+another, "reference" DNS. It supports the following command-line flags:
 
 * `-H name`/`--hostname name`: the host name or address of the server to
   check.
@@ -49,6 +49,48 @@ supports the following command-line flags:
 * `-p port`/`--rs-port port`: the port to use on the reference server
   (defaults to 53).
 
+### Generic text match counter
+
+The `check_output_matches` plugin can be used to count occurrences of strings
+in a program's output or in a text file, and compute its final status based on
+that.
+
+It supports the following general command line flags:
+
+* `-f` / `--is-file` indicates that the plugin will be reading from a text file
+  instead of running another program;
+* `-s` / `--source` is either the name of the file to read, or the command to
+  execute. The command may include multiple arguments separated by single
+  spaces; it does not support any form of quoting.
+* `-T` / `--timeout` can set a timeout for the command. It is disabled by
+  default.
+* `-S` / `--strict` determines how unmatched lines are handled. By default they
+  are ignored, but setting this flag will cause the plugin to enter `CRITICAL`
+  mode if unmatched lines are found.
+
+Other flags are available in order to configure the matches. The main flag is
+`-m` / `--match`, which adds a new match string to the set of checks to run.
+The checks are influenced by the following additional flags, which apply to all
+subsequent matches.
+
+* `-r` / `--regexp` indicates that new matches will be based on regular
+  expressions instead of substrings.
+* `-R` / `--no-regexp` switches back to substring-based matches.
+* `-w` / `--warn` can be used to set a warning range. It must be followed by
+  a Nagios range specification.
+* `-W` / `--no-warn` clears the warning range.
+* `-c` / `--critical` can be used to set the critical range. It must be followed
+  by a Nagios range specification.
+* `-C` / `--no-critical` clears the critical range.
+
+For example, the command below:
+
+    gomonop check_output_matches -s 'find /some/place' \
+       -w 4 -r -m '^.*\.warn$' \
+       -W -c 0 -R -m fatal
+
+configures a warning if more than 4 files ending in `.warn` are found, and a
+critical state if any file with `fatal` in its name is found.
 
 Building from source
 --------------------
diff --git a/cmd/matches/cmdline.go b/cmd/matches/cmdline.go
new file mode 100644
index 0000000..fbf9e3b
--- /dev/null
+++ b/cmd/matches/cmdline.go
@@ -0,0 +1,95 @@
+package matches // import nocternity.net/gomonop/cmd/matches
+
+import (
+	"context"
+	"regexp"
+	"strings"
+	"time"
+
+	"github.com/karrick/golf"
+
+	"nocternity.net/gomonop/pkg/perfdata"
+)
+
+// pluginFlags represent command line flags that have been parsed.
+type pluginFlags struct {
+	isFile     bool          // Are we reading from a file?
+	dataSource string        // The file or command to read from
+	timeout    time.Duration // A timeout for the command, or 0 to disable
+	matches    []matchConfig // Configuration for the matches to check
+	strict     bool          // Reject lines that don't match anything
+}
+
+// matchConfig is the configuration for a single match to check.
+type matchConfig struct {
+	isRegexp    bool            // Are we checking against a regular expression?
+	matchString string          // The string or regexp to match
+	compiledRe  *regexp.Regexp  // The compiled regexp
+	warn        *perfdata.Range // Warning range
+	crit        *perfdata.Range // Critical range
+}
+
+// parseArguments parses command line arguments for the plugin.
+func (p *pluginFlags) parseArguments() {
+	golf.BoolVarP(&p.isFile, 'f', "is-file", false, "Are we reading from a file?")
+	golf.StringVarP(&p.dataSource, 's', "source", "", "The file or command to read from")
+	golf.DurationVarP(&p.timeout, 'T', "timeout", 0, "A timeout for the command, or 0 to disable")
+	golf.BoolVarP(&p.strict, 'S', "strict", false, "Reject lines that do not match anything")
+
+	isRegexp := golf.BoolP('R', "no-regexp", true, "Following match argument will be a basic string")
+	golf.BoolVarP(isRegexp, 'r', "regexp", false, "Following match argument will be a regexp")
+
+	var wRange *perfdata.Range
+	golf.StringFuncP('w', "warn", "", "Warning range, in Nagios-compatible format", func(s string) error {
+		locRange, err := perfdata.ParseRange(s)
+		if err == nil {
+			wRange = locRange
+		}
+		return err
+	})
+	golf.BoolFuncP('W', "no-warn", false, "Clear warning range", func(bool) error {
+		wRange = nil
+		return nil
+	})
+
+	var cRange *perfdata.Range
+	golf.StringFuncP('c', "critical", "", "Critical range, in Nagios-compatible format", func(s string) error {
+		locRange, err := perfdata.ParseRange(s)
+		if err == nil {
+			cRange = locRange
+		}
+		return err
+	})
+	golf.BoolFuncP('C', "no-critical", false, "Clear warning range", func(bool) error {
+		cRange = nil
+		return nil
+	})
+
+	golf.StringFuncP('m', "match", "", "Match string", func(s string) error {
+		p.matches = append(p.matches, matchConfig{
+			isRegexp:    *isRegexp,
+			matchString: s,
+			warn:        wRange,
+			crit:        cRange,
+		})
+		return nil
+	})
+
+	golf.Parse()
+}
+
+// makeContext generates a context based on the timeout, if one is set.
+func (p *pluginFlags) makeContext() (context.Context, context.CancelFunc) {
+	if p.timeout == 0 {
+		return context.Background(), func() {}
+	}
+	return context.WithTimeout(context.Background(), p.timeout)
+}
+
+// matches check if the specified string matches a configuration.
+func (p *matchConfig) matches(s string) bool {
+	if p.isRegexp {
+		return p.compiledRe.MatchString(s)
+	}
+	return strings.Contains(s, p.matchString)
+}
diff --git a/cmd/matches/extprogram.go b/cmd/matches/extprogram.go
new file mode 100644
index 0000000..aa45dee
--- /dev/null
+++ b/cmd/matches/extprogram.go
@@ -0,0 +1,118 @@
+package matches // import  nocternity.net/gomonop/pkg/matches
+
+import (
+	"bufio"
+	"context"
+	"io"
+	"os/exec"
+	"strings"
+)
+
+// streamObtainer refers to functions that may return either an input stream or
+// an error.
+type streamObtainer func() (io.ReadCloser, error)
+
+// pipeCopy encapsulates the structures that are used to transfer data from a
+// single stream to the reader.
+type pipeCopy struct {
+	reader     io.ReadCloser // The input stream being read from
+	dataPipe   chan string   // Channel that receives the data we read
+	aborter    chan struct{} // Channel that causes the copy to abort
+	readResult chan error    // Channel that returns results from reading
+}
+
+// startPipeCopy starts a goroutine that copies data from the reader to the
+// dataPipe.
+func startPipeCopy(obtainer streamObtainer, pipes *readerPipes) *pipeCopy {
+	stream, err := obtainer()
+	if err != nil {
+		pipes.done <- err
+		return nil
+	}
+
+	pipeCopyData := &pipeCopy{
+		reader:     stream,
+		dataPipe:   pipes.data,
+		aborter:    make(chan struct{}),
+		readResult: make(chan error),
+	}
+
+	go pipeCopyData.run()
+
+	return pipeCopyData
+}
+
+// run runs the copy operation from the input stream to the data pipe. It is
+// meant to be executed as a goroutine.
+func (pc *pipeCopy) run() {
+	scanner := bufio.NewScanner(pc.reader)
+	for scanner.Scan() {
+		select {
+		case <-pc.aborter:
+			return
+		case pc.dataPipe <- scanner.Text():
+		}
+	}
+	pc.readResult <- scanner.Err()
+}
+
+// close closes the stream and channels used by a copy pipe.
+func (pc *pipeCopy) close() {
+	pc.abort()
+	_ = pc.reader.Close()
+	close(pc.aborter)
+	close(pc.readResult)
+}
+
+// abort causes the pipe copy to be aborted.
+func (pc *pipeCopy) abort() {
+	pc.aborter <- struct{}{}
+}
+
+// readFromProgram starts a goroutine that controls the program, sending lines
+// from both stderr and stdout to the dataPipe.
+func (pluginInst *matchesPlugin) readFromProgram(ctx context.Context, pipes *readerPipes) {
+	go func() {
+		args := strings.Split(pluginInst.dataSource, " ")
+		cmd := exec.Command(args[0], args[1:]...) //nolint:gosec // Command is in fact user-provided
+
+		outs := startPipeCopy(func() (io.ReadCloser, error) { return cmd.StdoutPipe() }, pipes)
+		defer outs.close()
+		errs := startPipeCopy(func() (io.ReadCloser, error) { return cmd.StderrPipe() }, pipes)
+		defer errs.close()
+
+		if err := cmd.Start(); err != nil {
+			pipes.done <- err
+			return
+		}
+
+		abort := func(err error) {
+			_ = cmd.Process.Kill()
+			pipes.done <- err
+		}
+
+		errComplete := false
+		outComplete := false
+		for !(errComplete && outComplete) {
+			select {
+			case <-ctx.Done():
+				abort(ctx.Err())
+				return
+			case err := <-outs.readResult:
+				if err != nil {
+					abort(err)
+					return
+				}
+				outComplete = true
+			case err := <-errs.readResult:
+				if err != nil {
+					abort(err)
+					return
+				}
+				errComplete = true
+			}
+		}
+
+		pipes.done <- cmd.Wait()
+	}()
+}
diff --git a/cmd/matches/plugin.go b/cmd/matches/plugin.go
new file mode 100644
index 0000000..8f56a30
--- /dev/null
+++ b/cmd/matches/plugin.go
@@ -0,0 +1,76 @@
+// The matches package contains the implementation of the `check_output_matches`
+// plugin, which can be used to run an arbitrary command and check its output
+// against various patterns. It can also be configured to do the same on an
+// arbitrary file.
+package matches // import nocternity.net/gomonop/cmd/matches
+
+import (
+	"regexp"
+
+	"nocternity.net/gomonop/pkg/plugin"
+	"nocternity.net/gomonop/pkg/results"
+	"nocternity.net/gomonop/pkg/status"
+)
+
+// Program data for the string matches plugin.
+type matchesPlugin struct {
+	pluginFlags                     // Flags from the command line
+	results        *results.Results // Plugin output state
+	counters       []int            // Counters for each match
+	unmatchedLines int              // Number of lines that didn't match anything
+}
+
+// Initialise the plugin.
+func NewPlugin() plugin.Plugin {
+	pluginInst := &matchesPlugin{
+		results: results.New("String matches counter"),
+	}
+	pluginInst.parseArguments()
+	return pluginInst
+}
+
+// Return the program's output value.
+func (pluginInst *matchesPlugin) Results() *results.Results {
+	return pluginInst.results
+}
+
+// Check the values that were specified from the command line. Returns true
+// if the arguments made sense.
+func (pluginInst *matchesPlugin) CheckArguments() bool {
+	if pluginInst.dataSource == "" {
+		pluginInst.results.SetState(status.StatusUnknown, "no data source specified")
+		return false
+	}
+
+	if !pluginInst.strict && len(pluginInst.matches) == 0 {
+		pluginInst.results.SetState(status.StatusUnknown, "would match anything")
+		return false
+	}
+
+	for index := range pluginInst.matches {
+		if pluginInst.matches[index].matchString == "" {
+			pluginInst.results.SetState(status.StatusUnknown, "empty match string")
+			pluginInst.results.AddLinef("(At match %d)", index+1)
+			return false
+		}
+		if pluginInst.matches[index].isRegexp {
+			rexp, err := regexp.Compile(pluginInst.matches[index].matchString)
+			if err != nil {
+				pluginInst.results.SetState(status.StatusUnknown, "Invalid regular expression")
+				pluginInst.results.AddLine(err.Error())
+				pluginInst.results.AddLinef("(At match %d)", index+1)
+				return false
+			}
+			pluginInst.matches[index].compiledRe = rexp
+		}
+	}
+
+	return true
+}
+
+// Run the check.
+func (pluginInst *matchesPlugin) RunCheck() {
+	pluginInst.counters = make([]int, len(pluginInst.matches))
+	err := pluginInst.processData()
+	pluginInst.checkResults(err)
+}
diff --git a/cmd/matches/reader.go b/cmd/matches/reader.go
new file mode 100644
index 0000000..87d15b1
--- /dev/null
+++ b/cmd/matches/reader.go
@@ -0,0 +1,95 @@
+package matches // import  nocternity.net/gomonop/pkg/matches
+
+import (
+	"bufio"
+	"context"
+	"os"
+)
+
+// readerPipes encapsulates the two channels used to communicate data and state
+// from the functions that acquire the data.
+type readerPipes struct {
+	done chan error
+	data chan string
+}
+
+// makeReaderPipes initializes the channels.
+func makeReaderPipes() *readerPipes {
+	return &readerPipes{
+		done: make(chan error),
+		data: make(chan string),
+	}
+}
+
+// close closes the channels.
+func (rdp *readerPipes) close() {
+	close(rdp.done)
+	close(rdp.data)
+}
+
+// readFromFile starts a goroutine that reads from the file and sends each line
+// to the dataPipe.
+func (pluginInst *matchesPlugin) readFromFile(ctx context.Context, pipes *readerPipes) {
+	go func() {
+		file, err := os.Open(pluginInst.dataSource)
+		if err != nil {
+			pipes.done <- err
+			return
+		}
+		defer file.Close()
+
+		scanner := bufio.NewScanner(file)
+		for scanner.Scan() {
+			select {
+			case <-ctx.Done():
+				pipes.done <- ctx.Err()
+				return
+			case pipes.data <- scanner.Text():
+			}
+		}
+		pipes.done <- scanner.Err()
+	}()
+}
+
+// startReading starts reading data from either a file or a program.
+func (pluginInst *matchesPlugin) startReading(ctx context.Context, pipes *readerPipes) {
+	if pluginInst.isFile {
+		pluginInst.readFromFile(ctx, pipes)
+	} else {
+		pluginInst.readFromProgram(ctx, pipes)
+	}
+}
+
+// processLine processes a single line obtained from the program or file.
+func (pluginInst *matchesPlugin) processLine(line string) {
+	hadMatch := false
+	for index := range pluginInst.matches {
+		if pluginInst.matches[index].matches(line) {
+			pluginInst.counters[index]++
+			hadMatch = true
+		}
+	}
+	if !hadMatch {
+		pluginInst.unmatchedLines++
+	}
+}
+
+// processData processes data from either the program or the file.
+func (pluginInst *matchesPlugin) processData() error {
+	ctx, cancel := pluginInst.makeContext()
+	defer cancel()
+
+	pipes := makeReaderPipes()
+	defer pipes.close()
+
+	pluginInst.startReading(ctx, pipes)
+
+	for {
+		select {
+		case line := <-pipes.data:
+			pluginInst.processLine(line)
+		case err := <-pipes.done:
+			return err
+		}
+	}
+}
diff --git a/cmd/matches/results.go b/cmd/matches/results.go
new file mode 100644
index 0000000..57cf4d3
--- /dev/null
+++ b/cmd/matches/results.go
@@ -0,0 +1,125 @@
+package matches // import nocternity.net/gomonop/cmd/matches
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+
+	"nocternity.net/gomonop/pkg/perfdata"
+	"nocternity.net/gomonop/pkg/status"
+)
+
+// resultsInfo contains information about results being processed.
+type resultsInfo struct {
+	nWarns, nCrits, nUnknowns uint
+}
+
+// updateResults updates the result counters based on the specified status.
+func (info *resultsInfo) updateFrom(itemStatus status.Status) {
+	switch itemStatus {
+	case status.StatusCritical:
+		info.nCrits++
+	case status.StatusWarning:
+		info.nWarns++
+	case status.StatusUnknown:
+		info.nUnknowns++
+	case status.StatusOK:
+		// do nothing
+	}
+}
+
+// toProblems generate the list of problems found while running the plugin,
+// as a string which can be returned to the monitoring server.
+func (info *resultsInfo) toProblems() string {
+	problems := []string{}
+	if info.nCrits > 0 {
+		problems = append(problems,
+			fmt.Sprintf("%d value(s) critical", info.nCrits),
+		)
+	}
+	if info.nWarns > 0 {
+		problems = append(problems,
+			fmt.Sprintf("%d value(s) warning", info.nWarns),
+		)
+	}
+	if info.nUnknowns > 0 {
+		problems = append(problems,
+			fmt.Sprintf("%d value(s) unknown", info.nUnknowns),
+		)
+	}
+	if len(problems) == 0 {
+		problems = append(problems, "No match errors")
+	}
+	return strings.Join(problems, ", ")
+}
+
+// toStatus computes the final status of the plugin based on the status
+// counters.
+func (info *resultsInfo) toStatus() status.Status {
+	switch {
+	case info.nCrits > 0:
+		return status.StatusCritical
+	case info.nWarns > 0:
+		return status.StatusWarning
+	case info.nUnknowns > 0:
+		return status.StatusUnknown
+	default:
+		return status.StatusOK
+	}
+}
+
+// checkCounters checks the match counters against their configured thresholds,
+// if any, and adds the corresponding perf data to the result. It initializes a
+// resultsInfo structure containing the counts of critical, warning and unknown
+// statuses gathered so far.
+func (pluginInst *matchesPlugin) checkCounters() resultsInfo {
+	info := resultsInfo{}
+
+	for index := range pluginInst.counters {
+		config := &pluginInst.matches[index]
+
+		var nature string
+		if config.isRegexp {
+			nature = "regexp"
+		} else {
+			nature = "substring"
+		}
+		label := fmt.Sprintf("#%2d : %s %s", index+1, nature, config.matchString)
+		value := strconv.Itoa(pluginInst.counters[index])
+
+		pdat := perfdata.New(label, perfdata.UomNone, value)
+		pdat.SetWarn(config.warn)
+		pdat.SetCrit(config.crit)
+		pluginInst.results.AddPerfData(pdat)
+
+		info.updateFrom(pdat.GetStatus())
+	}
+
+	return info
+}
+
+// checkUnmatched checks the unmatched lines counters, applying a limit if
+// strict mode is enabled. The corresponding performance data is added to the
+// result.
+func (pluginInst *matchesPlugin) checkUnmatched(info *resultsInfo) {
+	umlPdat := perfdata.New("unmatched", perfdata.UomNone, strconv.Itoa(pluginInst.unmatchedLines))
+	if pluginInst.strict {
+		umlPdat.SetCrit(perfdata.RangeMinMax("~", "0"))
+	}
+	info.updateFrom(umlPdat.GetStatus())
+	pluginInst.results.AddPerfData(umlPdat)
+}
+
+// checkResults checks the various counters against their configured thresholds,
+// and the strict mode failure count.
+func (pluginInst *matchesPlugin) checkResults(readError error) {
+	info := pluginInst.checkCounters()
+	pluginInst.checkUnmatched(&info)
+
+	if readError != nil {
+		pluginInst.results.AddLinef("Error while reading data: %s", readError.Error())
+		info.nUnknowns++
+	}
+
+	pluginInst.results.SetState(info.toStatus(), info.toProblems())
+}
diff --git a/cmd/sslcert/main.go b/cmd/sslcert/main.go
index eb7b2f1..87f1a34 100644
--- a/cmd/sslcert/main.go
+++ b/cmd/sslcert/main.go
@@ -17,6 +17,7 @@ import (
 	"nocternity.net/gomonop/pkg/perfdata"
 	"nocternity.net/gomonop/pkg/plugin"
 	"nocternity.net/gomonop/pkg/results"
+	"nocternity.net/gomonop/pkg/status"
 )
 
 //--------------------------------------------------------------------------------------------------------
@@ -224,20 +225,20 @@ func (program *checkProgram) Results() *results.Results {
 // if the arguments made sense.
 func (program *checkProgram) CheckArguments() bool {
 	if program.hostname == "" {
-		program.plugin.SetState(results.StatusUnknown, "no hostname specified")
+		program.plugin.SetState(status.StatusUnknown, "no hostname specified")
 		return false
 	}
 	if program.port < 1 || program.port > 65535 {
-		program.plugin.SetState(results.StatusUnknown, "invalid or missing port number")
+		program.plugin.SetState(status.StatusUnknown, "invalid or missing port number")
 		return false
 	}
 	if program.warn != -1 && program.crit != -1 && program.warn <= program.crit {
-		program.plugin.SetState(results.StatusUnknown, "nonsensical thresholds")
+		program.plugin.SetState(status.StatusUnknown, "nonsensical thresholds")
 		return false
 	}
 	if _, ok := certGetters[program.startTLS]; !ok {
 		errstr := "unsupported StartTLS protocol " + program.startTLS
-		program.plugin.SetState(results.StatusUnknown, errstr)
+		program.plugin.SetState(status.StatusUnknown, errstr)
 		return false
 	}
 	program.hostname = strings.ToLower(program.hostname)
@@ -262,13 +263,13 @@ func (program *checkProgram) getCertificate() error {
 // matches the requested host name.
 func (program *checkProgram) checkSANlessCertificate() bool {
 	if !program.ignoreCnOnly || len(program.extraNames) != 0 {
-		program.plugin.SetState(results.StatusWarning,
+		program.plugin.SetState(status.StatusWarning,
 			"certificate doesn't have SAN domain names")
 		return false
 	}
 	dn := strings.ToLower(program.certificate.Subject.String())
 	if !strings.HasPrefix(dn, fmt.Sprintf("cn=%s,", program.hostname)) {
-		program.plugin.SetState(results.StatusCritical, "incorrect certificate CN")
+		program.plugin.SetState(status.StatusCritical, "incorrect certificate CN")
 		return false
 	}
 	return true
@@ -298,7 +299,7 @@ func (program *checkProgram) checkNames() bool {
 		certificateIsOk = program.checkHostName(name) && certificateIsOk
 	}
 	if !certificateIsOk {
-		program.plugin.SetState(results.StatusCritical, "names missing from SAN domain names")
+		program.plugin.SetState(status.StatusCritical, "names missing from SAN domain names")
 	}
 	return certificateIsOk
 }
@@ -306,26 +307,26 @@ func (program *checkProgram) checkNames() bool {
 // Check a certificate's time to expiry against the warning and critical
 // thresholds, returning a status code and description based on these
 // values.
-func (program *checkProgram) checkCertificateExpiry(tlDays int) (results.Status, string) {
+func (program *checkProgram) checkCertificateExpiry(tlDays int) (status.Status, string) {
 	if tlDays <= 0 {
-		return results.StatusCritical, "certificate expired"
+		return status.StatusCritical, "certificate expired"
 	}
 
 	var limitStr string
-	var state results.Status
+	var state status.Status
 
 	switch {
 	case program.crit > 0 && tlDays <= program.crit:
 		limitStr = fmt.Sprintf(" (<= %d)", program.crit)
-		state = results.StatusCritical
+		state = status.StatusCritical
 
 	case program.warn > 0 && tlDays <= program.warn:
 		limitStr = fmt.Sprintf(" (<= %d)", program.warn)
-		state = results.StatusWarning
+		state = status.StatusWarning
 
 	default:
 		limitStr = ""
-		state = results.StatusOK
+		state = status.StatusOK
 	}
 
 	statusString := fmt.Sprintf("certificate will expire in %d days%s",
@@ -351,7 +352,7 @@ func (program *checkProgram) setPerfData(tlDays int) {
 func (program *checkProgram) RunCheck() {
 	err := program.getCertificate()
 	if err != nil {
-		program.plugin.SetState(results.StatusUnknown, err.Error())
+		program.plugin.SetState(status.StatusUnknown, err.Error())
 	} else if program.checkNames() {
 		timeLeft := time.Until(program.certificate.NotAfter)
 		tlDays := int((timeLeft + 86399*time.Second) / (24 * time.Hour))
diff --git a/cmd/zoneserial/main.go b/cmd/zoneserial/main.go
index f5a1f25..b73965c 100644
--- a/cmd/zoneserial/main.go
+++ b/cmd/zoneserial/main.go
@@ -15,6 +15,7 @@ import (
 	"nocternity.net/gomonop/pkg/perfdata"
 	"nocternity.net/gomonop/pkg/plugin"
 	"nocternity.net/gomonop/pkg/results"
+	"nocternity.net/gomonop/pkg/status"
 )
 
 //-------------------------------------------------------------------------------------------------------
@@ -93,23 +94,23 @@ func (program *checkProgram) Results() *results.Results {
 // Check the values that were specified from the command line. Returns true if the arguments made sense.
 func (program *checkProgram) CheckArguments() bool {
 	if program.hostname == "" {
-		program.plugin.SetState(results.StatusUnknown, "no DNS hostname specified")
+		program.plugin.SetState(status.StatusUnknown, "no DNS hostname specified")
 		return false
 	}
 	if program.port < 1 || program.port > 65535 {
-		program.plugin.SetState(results.StatusUnknown, "invalid DNS port number")
+		program.plugin.SetState(status.StatusUnknown, "invalid DNS port number")
 		return false
 	}
 	if program.zone == "" {
-		program.plugin.SetState(results.StatusUnknown, "no DNS zone specified")
+		program.plugin.SetState(status.StatusUnknown, "no DNS zone specified")
 		return false
 	}
 	if program.rsHostname == "" {
-		program.plugin.SetState(results.StatusUnknown, "no reference DNS hostname specified")
+		program.plugin.SetState(status.StatusUnknown, "no reference DNS hostname specified")
 		return false
 	}
 	if program.rsPort < 1 || program.rsPort > 65535 {
-		program.plugin.SetState(results.StatusUnknown, "invalid reference DNS port number")
+		program.plugin.SetState(status.StatusUnknown, "invalid reference DNS port number")
 		return false
 	}
 	program.hostname = strings.ToLower(program.hostname)
@@ -176,12 +177,12 @@ func (program *checkProgram) RunCheck() {
 	cOk, cSerial := program.getSerial("checked", checkResponse)
 	rOk, rSerial := program.getSerial("reference", refResponse)
 	if !(cOk && rOk) {
-		program.plugin.SetState(results.StatusUnknown, "could not read serials")
+		program.plugin.SetState(status.StatusUnknown, "could not read serials")
 		return
 	}
 	if cSerial == rSerial {
-		program.plugin.SetState(results.StatusOK, "serials match")
+		program.plugin.SetState(status.StatusOK, "serials match")
 	} else {
-		program.plugin.SetState(results.StatusCritical, "serials mismatch")
+		program.plugin.SetState(status.StatusCritical, "serials mismatch")
 	}
 }
diff --git a/go.mod b/go.mod
index 111d2ca..6b14533 100644
--- a/go.mod
+++ b/go.mod
@@ -5,14 +5,16 @@ go 1.22
 require (
 	github.com/karrick/golf v1.4.0
 	github.com/miekg/dns v1.1.40
+	github.com/stretchr/testify v1.9.0
 )
 
 require (
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
-	github.com/stretchr/testify v1.9.0 // indirect
 	golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550 // indirect
 	golang.org/x/net v0.0.0-20190923162816-aa69164e4478 // indirect
 	golang.org/x/sys v0.0.0-20190924154521-2837fb4f24fe // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )
+
+replace github.com/karrick/golf v1.4.0 => github.com/tseeker/golf v0.0.0-20240720130627-ce082c3b50d5
diff --git a/go.sum b/go.sum
index 37c20f7..d53fb3f 100644
--- a/go.sum
+++ b/go.sum
@@ -1,13 +1,13 @@
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/karrick/golf v1.4.0 h1:9i9HnUh7uCyUFJhIqg311HBibw4f2pbGldi0ZM2FhaQ=
-github.com/karrick/golf v1.4.0/go.mod h1:qGN0IhcEL+IEgCXp00RvH32UP59vtwc8w5YcIdArNRk=
 github.com/miekg/dns v1.1.40 h1:pyyPFfGMnciYUk/mXpKkVmeMQjfXqt3FAJ2hy7tPiLA=
 github.com/miekg/dns v1.1.40/go.mod h1:KNUDUusw/aVsxyTYZM1oqvCicbwhgbNgztCETuNZ7xM=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/stretchr/testify v1.9.0 h1:HtqpIVDClZ4nwg75+f6Lvsy/wHu+3BoSGCbBAcpTsTg=
 github.com/stretchr/testify v1.9.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
+github.com/tseeker/golf v0.0.0-20240720130627-ce082c3b50d5 h1:IreFtDkjJg81heDw+xIEkVZXySV4zQzwKbpenJBQHqk=
+github.com/tseeker/golf v0.0.0-20240720130627-ce082c3b50d5/go.mod h1:qGN0IhcEL+IEgCXp00RvH32UP59vtwc8w5YcIdArNRk=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550 h1:ObdrDkeb4kJdCP557AjRjq69pTHfNouLtWZG7j9rPN8=
 golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
@@ -25,6 +25,7 @@ golang.org/x/sys v0.0.0-20190924154521-2837fb4f24fe/go.mod h1:h1NjWce9XRLGQEsW7w
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/tools v0.0.0-20191216052735-49a3e744a425/go.mod h1:TB2adYChydJhpapKDTa4BR/hXlZSLoq2Wpct/0txZ28=
 golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
diff --git a/main.go b/main.go
index d535144..cf84934 100644
--- a/main.go
+++ b/main.go
@@ -5,15 +5,17 @@ import (
 	"os"
 	"path/filepath"
 
+	"nocternity.net/gomonop/cmd/matches"
 	"nocternity.net/gomonop/cmd/sslcert"
 	"nocternity.net/gomonop/cmd/zoneserial"
 	"nocternity.net/gomonop/pkg/plugin"
-	"nocternity.net/gomonop/pkg/results"
+	"nocternity.net/gomonop/pkg/status"
 	"nocternity.net/gomonop/pkg/version"
 )
 
 var (
 	plugins map[string]plugin.Builder = map[string]plugin.Builder{
+		"check_output_matches":  matches.NewPlugin,
 		"check_ssl_certificate": sslcert.NewProgram,
 		"check_zone_serial":     zoneserial.NewProgram,
 	}
@@ -28,7 +30,7 @@ func getPlugin() plugin.Plugin {
 
 	if len(os.Args) < 2 {
 		fmt.Printf("Syntax: %s <plugin> [arguments]\n", ownName)
-		fmt.Printf("        %s --plugin|-p\n", ownName)
+		fmt.Printf("        %s --plugins|-p\n", ownName)
 		fmt.Printf("        %s --version|-v\n", ownName)
 		os.Exit(1)
 	}
@@ -61,7 +63,7 @@ func main() {
 	output := runPlugin.Results()
 	defer func() {
 		if r := recover(); r != nil {
-			output.SetState(results.StatusUnknown, "Internal error")
+			output.SetState(status.StatusUnknown, "Internal error")
 			output.AddLinef("Error info: %v", r)
 		}
 		fmt.Println(output.String())
diff --git a/pkg/perfdata/perfdata.go b/pkg/perfdata/perfdata.go
index fc1914e..f551c78 100644
--- a/pkg/perfdata/perfdata.go
+++ b/pkg/perfdata/perfdata.go
@@ -3,7 +3,10 @@
 package perfdata // import nocternity.net/gomonop/pkg/perfdata
 
 import (
+	"strconv"
 	"strings"
+
+	"nocternity.net/gomonop/pkg/status"
 )
 
 // Performance data, including a label, units, a value, warning/critical
@@ -35,14 +38,22 @@ func New(label string, units UnitOfMeasurement, value string) *PerfData {
 
 // Set the warning range for the performance data record.
 func (d *PerfData) SetWarn(r *Range) {
-	d.warn = *r
-	d.bits |= PDatWarn
+	if r == nil {
+		d.bits &^= PDatWarn
+	} else {
+		d.warn = *r
+		d.bits |= PDatWarn
+	}
 }
 
 // Set the critical range for the performance data record.
 func (d *PerfData) SetCrit(r *Range) {
-	d.crit = *r
-	d.bits |= PDatCrit
+	if r == nil {
+		d.bits &^= PDatCrit
+	} else {
+		d.crit = *r
+		d.bits |= PDatCrit
+	}
 }
 
 // Set the performance data's minimal value.
@@ -63,6 +74,23 @@ func (d *PerfData) SetMax(max string) {
 	d.bits |= PDatMax
 }
 
+// Check the performance data's value against its configured ranges.
+func (d *PerfData) GetStatus() status.Status {
+	value, err := strconv.ParseFloat(d.value, 64)
+	if err != nil {
+		return status.StatusUnknown
+	}
+
+	if d.bits&PDatCrit != 0 && d.crit.Contains(value) {
+		return status.StatusCritical
+	}
+	if d.bits&PDatWarn != 0 && d.warn.Contains(value) {
+		return status.StatusWarning
+	}
+
+	return status.StatusOK
+}
+
 // Converts performance data to a string which may be read by the monitoring
 // system.
 func (d *PerfData) String() string {
diff --git a/pkg/perfdata/perfdata_test.go b/pkg/perfdata/perfdata_test.go
index 894077c..ac3f3e9 100644
--- a/pkg/perfdata/perfdata_test.go
+++ b/pkg/perfdata/perfdata_test.go
@@ -6,6 +6,7 @@ import (
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"nocternity.net/gomonop/pkg/status"
 )
 
 func TestNewNoValue(t *testing.T) {
@@ -50,6 +51,12 @@ func TestSetWarn(t *testing.T) {
 	assert.Equal(t, rangeStr, data.warn.String())
 }
 
+func TestSetWarnNil(t *testing.T) {
+	data := PerfData{}
+	data.SetWarn(nil)
+	assert.Equal(t, perfDataBits(0), data.bits&PDatWarn)
+}
+
 func TestSetWarnTwice(t *testing.T) {
 	range1Value := Range{start: "A", end: "B"}
 	range2Value := Range{start: "C", end: "D"}
@@ -64,6 +71,16 @@ func TestSetWarnTwice(t *testing.T) {
 	assert.Equal(t, range2Str, data.warn.String())
 }
 
+func TestSetWarnClear(t *testing.T) {
+	range1Value := Range{start: "A", end: "B"}
+
+	data := PerfData{}
+	data.SetWarn(&range1Value)
+	data.SetWarn(nil)
+
+	assert.Equal(t, perfDataBits(0), data.bits&PDatWarn)
+}
+
 func TestSetCrit(t *testing.T) {
 	rangeValue := Range{start: "A", end: "B"}
 	rangeStr := rangeValue.String()
@@ -75,6 +92,12 @@ func TestSetCrit(t *testing.T) {
 	assert.Equal(t, rangeStr, data.crit.String())
 }
 
+func TestSetCritNil(t *testing.T) {
+	data := PerfData{}
+	data.SetCrit(nil)
+	assert.Equal(t, perfDataBits(0), data.bits&PDatCrit)
+}
+
 func TestSetCritTwice(t *testing.T) {
 	range1Value := Range{start: "A", end: "B"}
 	range2Value := Range{start: "C", end: "D"}
@@ -89,6 +112,16 @@ func TestSetCritTwice(t *testing.T) {
 	assert.Equal(t, range2Str, data.crit.String())
 }
 
+func TestSetCritClear(t *testing.T) {
+	range1Value := Range{start: "A", end: "B"}
+
+	data := PerfData{}
+	data.SetCrit(&range1Value)
+	data.SetCrit(nil)
+
+	assert.Equal(t, perfDataBits(0), data.bits&PDatCrit)
+}
+
 func TestSetMin(t *testing.T) {
 	const min = "100"
 
@@ -135,6 +168,35 @@ func TestSetMaxTwice(t *testing.T) {
 	assert.True(t, data.bits&PDatMax != 0)
 }
 
+func TestCheck(t *testing.T) {
+	warnRange := RangeMinMax("0", "10").Inside()
+	critRange := RangeMinMax("1", "9").Inside()
+
+	type Test struct {
+		in  string
+		out status.Status
+	}
+
+	tests := []Test{
+		{in: "moo", out: status.StatusUnknown},
+		{in: "-1", out: status.StatusOK},
+		{in: "0", out: status.StatusWarning},
+		{in: "1", out: status.StatusCritical},
+		{in: "9", out: status.StatusCritical},
+		{in: "10", out: status.StatusWarning},
+		{in: "11", out: status.StatusOK},
+	}
+	for _, test := range tests {
+		pdat := PerfData{
+			value: test.in,
+			bits:  PDatCrit | PDatWarn,
+			warn:  *warnRange,
+			crit:  *critRange,
+		}
+		assert.Equal(t, test.out, pdat.GetStatus())
+	}
+}
+
 func TestString(t *testing.T) {
 	type Test struct {
 		PerfData
diff --git a/pkg/perfdata/range.go b/pkg/perfdata/range.go
index e4ad2c7..7defac8 100644
--- a/pkg/perfdata/range.go
+++ b/pkg/perfdata/range.go
@@ -1,5 +1,10 @@
 package perfdata // import nocternity.net/gomonop/pkg/perfdata
 
+import (
+	"fmt"
+	"strconv"
+)
+
 // Performance data range.
 type Range struct {
 	start  string
@@ -29,6 +34,9 @@ func RangeMinMax(min, max string) *Range {
 		panic("invalid performance data range minimum value")
 	}
 	pdRange := &Range{}
+	if min == "~" {
+		min = ""
+	}
 	pdRange.start = min
 	pdRange.end = max
 	return pdRange
@@ -60,3 +68,26 @@ func (r *Range) String() string {
 
 	return inside + start + ":" + r.end
 }
+
+// Contains checks whether a numeric value is within the range.
+func (r *Range) Contains(value float64) bool {
+	var inStart, inEnd bool
+
+	if r.start != "" {
+		startValue, err := strconv.ParseFloat(r.start, 64)
+		if err != nil {
+			panic(fmt.Sprintf("invalid performance data range start value: %v", err))
+		}
+		inStart = value < startValue
+	}
+
+	if r.end != "" {
+		endValue, err := strconv.ParseFloat(r.end, 64)
+		if err != nil {
+			panic(fmt.Sprintf("invalid performance data range end value: %v", err))
+		}
+		inEnd = value > endValue
+	}
+
+	return (inStart || inEnd) != r.inside
+}
diff --git a/pkg/perfdata/range_test.go b/pkg/perfdata/range_test.go
index 26d1f21..9873395 100644
--- a/pkg/perfdata/range_test.go
+++ b/pkg/perfdata/range_test.go
@@ -1,6 +1,7 @@
 package perfdata // import nocternity.net/gomonop/pkg/perfdata
 
 import (
+	"fmt"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -40,6 +41,14 @@ func TestRangeMinMax(t *testing.T) {
 	assert.False(t, pdr.inside, "Inside flag should not be set")
 }
 
+func TestRangeMinMaxOpen(t *testing.T) {
+	min, max := "~", "456"
+	pdr := RangeMinMax(min, max)
+	assert.Equal(t, "", pdr.start, "Min value not clear in PerfDataRange")
+	assert.Equal(t, max, pdr.end, "Max value not copied to PerfDataRange")
+	assert.False(t, pdr.inside, "Inside flag should not be set")
+}
+
 func TestRangeInside(t *testing.T) {
 	pdr := &Range{}
 	pdr = pdr.Inside()
@@ -65,3 +74,53 @@ func TestRangeString(t *testing.T) {
 		assert.Equal(t, test.out, result, "Expected '%s', got '%s'", test.out, result)
 	}
 }
+
+func TestRangeContains(t *testing.T) {
+	type Test struct {
+		pdr    Range
+		value  float64
+		result bool
+	}
+
+	tests := []Test{
+		{pdr: Range{start: "0", end: "10"}, value: 0, result: false},
+		{pdr: Range{start: "0", end: "10"}, value: 10, result: false},
+		{pdr: Range{start: "0", end: "10"}, value: -1, result: true},
+		{pdr: Range{start: "0", end: "10"}, value: 11, result: true},
+		{pdr: Range{start: "", end: "10"}, value: -1000, result: false},
+		{pdr: Range{start: "", end: "10"}, value: 10, result: false},
+		{pdr: Range{start: "", end: "10"}, value: 11, result: true},
+		{pdr: Range{start: "10", end: ""}, value: -1000, result: true},
+		{pdr: Range{start: "10", end: ""}, value: 9, result: true},
+		{pdr: Range{start: "10", end: ""}, value: 10, result: false},
+		{pdr: Range{start: "10", end: "20"}, value: 9, result: true},
+		{pdr: Range{start: "10", end: "20"}, value: 10, result: false},
+		{pdr: Range{start: "10", end: "20"}, value: 20, result: false},
+		{pdr: Range{start: "10", end: "20"}, value: 21, result: true},
+	}
+
+	// Test cases with the inside flag set and the opposite result
+	n := len(tests)
+	for i := range n {
+		tests = append(tests, Test{
+			pdr: Range{
+				start:  tests[i].pdr.start,
+				end:    tests[i].pdr.end,
+				inside: !tests[i].pdr.inside,
+			},
+			value:  tests[i].value,
+			result: !tests[i].result,
+		})
+	}
+
+	for _, test := range tests {
+		t.Run(fmt.Sprintf("%v", test), func(t *testing.T) {
+			result := test.pdr.Contains(test.value)
+			assert.Equal(
+				t, test.result, result,
+				"Expected '%v', got '%v' for value '%f' and range '%s'",
+				test.result, result, test.value, test.pdr.String(),
+			)
+		})
+	}
+}
diff --git a/pkg/perfdata/rangeparser.go b/pkg/perfdata/rangeparser.go
new file mode 100644
index 0000000..c9ea0c1
--- /dev/null
+++ b/pkg/perfdata/rangeparser.go
@@ -0,0 +1,196 @@
+package perfdata // import nocternity.net/gomonop/pkg/perfdata
+
+import (
+	"fmt"
+	"strings"
+)
+
+// A state of the range parser.
+type rangeParserState int
+
+const (
+	rpsInit        rangeParserState = iota // Initial state
+	rpsExpectStart                         // Expect the start of the range
+	rpsInStart                             // Reading the start of the range
+	rpsExpectColon                         // Expect the colon that separates the start and end
+	rpsExpectEnd                           // Expect the end of the range
+	rpsInEnd                               // Reading the end of the range
+)
+
+// An error emitted by the range parser.
+type rangeParserError struct {
+	input    string
+	position int
+	message  string
+}
+
+// parseError creates a new range parser error.
+func parseError(input string, position int, message string) *rangeParserError {
+	return &rangeParserError{
+		input:    input,
+		position: position,
+		message:  message,
+	}
+}
+
+// Error implements the error interface for the range parser error type.
+func (rpe *rangeParserError) Error() string {
+	return fmt.Sprintf("in `%s' at position %d: %s", rpe.input, rpe.position, rpe.message)
+}
+
+// The full state of the range parser.
+type rangeParser struct {
+	runes        []rune           // The runes being parsed
+	index        int              // The read position
+	state        rangeParserState // The FSM state
+	startOfValue int              // The position of the start of the value being read
+	strBuilder   strings.Builder  // An accumulator for values
+	output       Range            // The output being generated
+}
+
+// initRangeParser initializes the range parser from the specified input.
+func initRangeParser(input string) *rangeParser {
+	return &rangeParser{
+		runes: []rune(input),
+		state: rpsInit,
+	}
+}
+
+// handleInit handles the parser's rpsInit state. It can accept a '@' or skip
+// to rpsExpectStart.
+func (rp *rangeParser) handleInit(current rune) {
+	if current == '@' {
+		rp.output.inside = true
+		rp.index++
+	}
+	rp.state = rpsExpectStart
+}
+
+// handleExpectStart handles the parser's rpsExpectStart state. It may accept
+// a colon, which will move the parser to the rpsExpectEnd state, a '~' which
+// will move it to rpsExpectColon, or any other rune which will cause the
+// accumulation of the "start" value to begin, and the state to be moved to
+// handleInStart.
+func (rp *rangeParser) handleExpectStart(current rune) {
+	switch current {
+	case ':':
+		rp.output.start = "0"
+		rp.state = rpsExpectEnd
+		rp.index++
+	case '~':
+		rp.state = rpsExpectColon
+		rp.index++
+	default:
+		rp.startOfValue = rp.index
+		rp.state = rpsInStart
+	}
+}
+
+// handleInStart handles the parser's rpsInStart state, which corresponds to the
+// reading of the "start" value. If a colon is found, the value will be written
+// to the range structure and validated, before switching to the rpsExpectEnd
+// state. Otherwise it will simply accumulate runes.
+func (rp *rangeParser) handleInStart(current rune) error {
+	switch current {
+	case ':':
+		rp.output.start = rp.strBuilder.String()
+		if !valueCheck.MatchString(rp.output.start) {
+			return parseError(string(rp.runes), rp.startOfValue, "invalid start value")
+		}
+		rp.strBuilder.Reset()
+		rp.state = rpsExpectEnd
+	default:
+		rp.strBuilder.WriteRune(current)
+	}
+
+	rp.index++
+	return nil
+}
+
+// handleExpectColon handles the parser's rpsExpectColon state. A colon MUST be
+// read, otherwise an error will be returned. The parser will then switch to the
+// rpsExpectEnd state.
+func (rp *rangeParser) handleExpectColon(current rune) error {
+	switch current {
+	case ':':
+		rp.state = rpsExpectEnd
+		rp.index++
+	default:
+		return parseError(string(rp.runes), rp.index, "expected ':'")
+	}
+
+	return nil
+}
+
+// handleExpectEnd handles the parser's rpsExpectEnd state, which sets the
+// position of the beginning of the "end" value, and jumps to the rpsInEnd
+// state.
+func (rp *rangeParser) handleExpectEnd() {
+	rp.startOfValue = rp.index
+	rp.state = rpsInEnd
+}
+
+// handleInEnd handles the parser's rpsInEnd state, which accumulates runes for
+// the "end" value.
+func (rp *rangeParser) handleInEnd(current rune) {
+	rp.strBuilder.WriteRune(current)
+	rp.index++
+}
+
+// consumeInput is the parser's state machine.
+func (rp *rangeParser) consumeInput() error {
+	for rp.index < len(rp.runes) {
+		var err error
+		curRune := rp.runes[rp.index]
+
+		switch rp.state {
+		case rpsInit:
+			rp.handleInit(curRune)
+
+		case rpsExpectStart:
+			rp.handleExpectStart(curRune)
+
+		case rpsInStart:
+			err = rp.handleInStart(curRune)
+
+		case rpsExpectColon:
+			err = rp.handleExpectColon(curRune)
+
+		case rpsExpectEnd:
+			rp.handleExpectEnd()
+
+		case rpsInEnd:
+			rp.handleInEnd(curRune)
+		}
+
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// Try to parse a string into a performance data range.
+func ParseRange(input string) (*Range, error) {
+	parser := initRangeParser(input)
+	if err := parser.consumeInput(); err != nil {
+		return nil, err
+	}
+
+	if parser.state == rpsInStart {
+		// The range was a single value, so that's the upper bound.
+		parser.output.start = "0"
+		parser.state = rpsInEnd
+	}
+	if parser.state != rpsInEnd {
+		return nil, parseError(input, parser.index, "unexpected end of input")
+	}
+
+	parser.output.end = parser.strBuilder.String()
+	if !valueCheck.MatchString(parser.output.end) {
+		return nil, parseError(input, parser.startOfValue, "invalid end value")
+	}
+
+	return &parser.output, nil
+}
diff --git a/pkg/perfdata/rangeparser_test.go b/pkg/perfdata/rangeparser_test.go
new file mode 100644
index 0000000..8ff5271
--- /dev/null
+++ b/pkg/perfdata/rangeparser_test.go
@@ -0,0 +1,66 @@
+package perfdata // import nocternity.net/gomonop/pkg/perfdata
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRangeParserOk(t *testing.T) {
+	type Test struct {
+		in  string
+		out Range
+	}
+	tests := []Test{
+		{in: ":0", out: Range{start: "0", end: "0"}},
+		{in: "0:0", out: Range{start: "0", end: "0"}},
+		{in: "~:0", out: Range{start: "", end: "0"}},
+		{in: ":123", out: Range{start: "0", end: "123"}},
+		{in: "0:123", out: Range{start: "0", end: "123"}},
+		{in: "~:123", out: Range{start: "", end: "123"}},
+		{in: "1", out: Range{start: "0", end: "1"}},
+		{in: "@:0", out: Range{start: "0", end: "0", inside: true}},
+		{in: "@0:0", out: Range{start: "0", end: "0", inside: true}},
+		{in: "@~:0", out: Range{start: "", end: "0", inside: true}},
+		{in: "@:123", out: Range{start: "0", end: "123", inside: true}},
+		{in: "@0:123", out: Range{start: "0", end: "123", inside: true}},
+		{in: "@~:123", out: Range{start: "", end: "123", inside: true}},
+		{in: "@1", out: Range{start: "0", end: "1", inside: true}},
+	}
+
+	for _, test := range tests {
+		result, err := ParseRange(test.in)
+		require.NoError(t, err, "Expected no error, got '%v'", err)
+		assert.Equal(t, test.out, *result, "Expected '%v', got '%v'", test.out, *result)
+	}
+}
+
+func TestRangeParserError(t *testing.T) {
+	type Test struct {
+		in     string
+		errPos int
+	}
+	tests := []Test{
+		{in: "", errPos: 0},
+		{in: ":", errPos: 1},
+		{in: "x:1", errPos: 0},
+		{in: ":~", errPos: 1},
+		{in: "@", errPos: 1},
+		{in: "@:", errPos: 2},
+		{in: "@x:1", errPos: 1},
+		{in: "@:~", errPos: 2},
+	}
+
+	for _, test := range tests {
+		result, err := ParseRange(test.in)
+		require.Error(t, err, "Expected error, got '%v'", err)
+		assert.Nil(t, result, "Expected nil result, got '%v'", result)
+		assert.True(
+			t, strings.Contains(err.Error(), fmt.Sprintf("at position %d", test.errPos)),
+			"Expected error to contain '%s', got '%s'", fmt.Sprintf("at position %d", test.errPos), err,
+		)
+	}
+}
diff --git a/pkg/results/results.go b/pkg/results/results.go
index 3118071..bc34c13 100644
--- a/pkg/results/results.go
+++ b/pkg/results/results.go
@@ -9,6 +9,7 @@ import (
 	"strings"
 
 	"nocternity.net/gomonop/pkg/perfdata"
+	"nocternity.net/gomonop/pkg/status"
 )
 
 // Results represents the monitoring plugin's results, including its name,
@@ -16,7 +17,7 @@ import (
 // data to be encoded in the output.
 type Results struct {
 	name      string
-	status    Status
+	status    status.Status
 	message   string
 	extraText *list.List
 	perfData  map[string]*perfdata.PerfData
@@ -26,7 +27,7 @@ type Results struct {
 func New(name string) *Results {
 	p := new(Results)
 	p.name = name
-	p.status = StatusUnknown
+	p.status = status.StatusUnknown
 	p.message = "no status set"
 	p.perfData = make(map[string]*perfdata.PerfData)
 	return p
@@ -34,7 +35,7 @@ func New(name string) *Results {
 
 // SetState sets the plugin's output code to `status` and its message to
 // the specified `message`.
-func (p *Results) SetState(status Status, message string) {
+func (p *Results) SetState(status status.Status, message string) {
 	p.status = status
 	p.message = message
 }
diff --git a/pkg/results/results_test.go b/pkg/results/results_test.go
index 32ddacf..b6566fb 100644
--- a/pkg/results/results_test.go
+++ b/pkg/results/results_test.go
@@ -5,14 +5,16 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+
 	"nocternity.net/gomonop/pkg/perfdata"
+	"nocternity.net/gomonop/pkg/status"
 )
 
 func TestNew(t *testing.T) {
 	p := New("test")
 
 	assert.Equal(t, p.name, "test")
-	assert.Equal(t, p.status, StatusUnknown)
+	assert.Equal(t, p.status, status.StatusUnknown)
 	assert.Equal(t, p.message, "no status set")
 	assert.Nil(t, p.extraText)
 	assert.NotNil(t, p.perfData)
@@ -21,9 +23,9 @@ func TestNew(t *testing.T) {
 func TestSetState(t *testing.T) {
 	p := Results{}
 
-	p.SetState(StatusWarning, "test")
+	p.SetState(status.StatusWarning, "test")
 
-	assert.Equal(t, p.status, StatusWarning)
+	assert.Equal(t, p.status, status.StatusWarning)
 	assert.Equal(t, p.message, "test")
 }
 
@@ -96,7 +98,7 @@ func TestString(t *testing.T) {
 		{
 			Results{
 				name:     "test",
-				status:   StatusWarning,
+				status:   status.StatusWarning,
 				message:  "test",
 				perfData: make(map[string]*perfdata.PerfData),
 			},
@@ -106,7 +108,7 @@ func TestString(t *testing.T) {
 			func() Results {
 				p := Results{
 					name:      "test",
-					status:    StatusWarning,
+					status:    status.StatusWarning,
 					message:   "test",
 					perfData:  make(map[string]*perfdata.PerfData),
 					extraText: list.New(),
@@ -121,7 +123,7 @@ func TestString(t *testing.T) {
 			func() Results {
 				p := Results{
 					name:     "test",
-					status:   StatusWarning,
+					status:   status.StatusWarning,
 					message:  "test",
 					perfData: make(map[string]*perfdata.PerfData),
 				}
@@ -136,7 +138,7 @@ func TestString(t *testing.T) {
 			func() Results {
 				p := Results{
 					name:      "test",
-					status:    StatusWarning,
+					status:    status.StatusWarning,
 					message:   "test",
 					perfData:  make(map[string]*perfdata.PerfData),
 					extraText: list.New(),
@@ -161,15 +163,15 @@ func TestString(t *testing.T) {
 func TestExitCode(t *testing.T) {
 	p := Results{}
 
-	p.status = StatusOK
-	assert.Equal(t, int(StatusOK), p.ExitCode())
+	p.status = status.StatusOK
+	assert.Equal(t, int(status.StatusOK), p.ExitCode())
 
-	p.status = StatusWarning
-	assert.Equal(t, int(StatusWarning), p.ExitCode())
+	p.status = status.StatusWarning
+	assert.Equal(t, int(status.StatusWarning), p.ExitCode())
 
-	p.status = StatusCritical
-	assert.Equal(t, int(StatusCritical), p.ExitCode())
+	p.status = status.StatusCritical
+	assert.Equal(t, int(status.StatusCritical), p.ExitCode())
 
-	p.status = StatusUnknown
-	assert.Equal(t, int(StatusUnknown), p.ExitCode())
+	p.status = status.StatusUnknown
+	assert.Equal(t, int(status.StatusUnknown), p.ExitCode())
 }
diff --git a/pkg/results/status.go b/pkg/status/status.go
similarity index 74%
rename from pkg/results/status.go
rename to pkg/status/status.go
index 6238a20..f34382e 100644
--- a/pkg/results/status.go
+++ b/pkg/status/status.go
@@ -1,4 +1,6 @@
-package results // import nocternity.net/gomonop/pkg/results
+// The status package contains the datatype that corresponds to monitoring
+// plugin status values.
+package status // import nocternity.net/gomonop/pkg/status
 
 // Status represents the return status of the monitoring plugin. The
 // corresponding integer value will be used as the program's exit code,
diff --git a/pkg/results/status_test.go b/pkg/status/status_test.go
similarity index 90%
rename from pkg/results/status_test.go
rename to pkg/status/status_test.go
index 8f81f43..0e3de96 100644
--- a/pkg/results/status_test.go
+++ b/pkg/status/status_test.go
@@ -1,4 +1,4 @@
-package results // import nocternity.net/gomonop/pkg/results
+package status // import nocternity.net/gomonop/pkg/status
 
 import (
 	"testing"