diff --git a/go.sum b/go.sum
index 6f35647..b5e2922 100644
--- a/go.sum
+++ b/go.sum
@@ -1,19 +1,2 @@
-github.com/BurntSushi/toml v0.3.1 h1:WXkYYl6Yr3qBf1K79EBnL4mak0OimBfB0XUf9Vl28OQ=
-github.com/BurntSushi/toml v0.3.1/go.mod h1:xHWCNGjB5oqiDr8zfno3MHue2Ht5sIBksp03qcyfWMU=
 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=
-gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
-gopkg.in/yaml.v2 v2.2.2 h1:ZCJp+EgiOT7lHqUV2J862kp8Qj64Jo6az82+3Td9dZw=
-gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
-gopkg.in/yaml.v2 v2.2.3 h1:fvjTMHxHEw/mxHbtzPi3JCcKXQRAnQTBRo6YCJSVHKI=
-gopkg.in/yaml.v2 v2.2.3/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
-gopkg.in/yaml.v2 v2.2.4 h1:/eiJrUcujPVeJ3xlSWaiNi3uSVmDGBK1pDHUHAnao1I=
-gopkg.in/yaml.v2 v2.2.4/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
-gopkg.in/yaml.v2 v2.2.5 h1:ymVxjfMaHvXD8RqPRmzHHsB3VvucivSkIAvJFDI5O3c=
-gopkg.in/yaml.v2 v2.2.5/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
-gopkg.in/yaml.v2 v2.2.7 h1:VUgggvou5XRW9mHwD/yXxIYSMtY0zoKQf/v226p2nyo=
-gopkg.in/yaml.v2 v2.2.7/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
-gopkg.in/yaml.v2 v2.2.8 h1:obN1ZagJSUGI0Ek/LBmuj4SNLPfIny3KsKFopxRdj10=
-gopkg.in/yaml.v2 v2.2.8/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
-gopkg.in/yaml.v2 v2.3.0 h1:clyUAQHOM3G0M3f5vQj7LuJrETvjVot3Z5el9nffUtU=
-gopkg.in/yaml.v2 v2.3.0/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
diff --git a/parser.go b/parser.go
index 7eb672e..f5e1a44 100644
--- a/parser.go
+++ b/parser.go
@@ -7,7 +7,6 @@ import (
 	"fmt"
 	"math"
 	"reflect"
-	"regexp"
 	"strconv"
 	"strings"
 	"time"
@@ -231,19 +230,38 @@ func (p *tomlParser) parseAssign() tomlParserStateFn {
 	return p.parseStart
 }
 
-var numberUnderscoreInvalidRegexp *regexp.Regexp
-var hexNumberUnderscoreInvalidRegexp *regexp.Regexp
+var errInvalidUnderscore = errors.New("invalid use of _ in number")
 
 func numberContainsInvalidUnderscore(value string) error {
-	if numberUnderscoreInvalidRegexp.MatchString(value) {
-		return errors.New("invalid use of _ in number")
+	// For large numbers, you may use underscores between digits to enhance
+	// readability. Each underscore must be surrounded by at least one digit on
+	// each side.
+
+	hasBefore := false
+	for idx, r := range value {
+		if r == '_' {
+			if !hasBefore || idx+1 >= len(value) {
+				// can't end with an underscore
+				return errInvalidUnderscore
+			}
+		}
+		hasBefore = isDigit(r)
 	}
 	return nil
 }
 
+var errInvalidUnderscoreHex = errors.New("invalid use of _ in hex number")
+
 func hexNumberContainsInvalidUnderscore(value string) error {
-	if hexNumberUnderscoreInvalidRegexp.MatchString(value) {
-		return errors.New("invalid use of _ in hex number")
+	hasBefore := false
+	for idx, r := range value {
+		if r == '_' {
+			if !hasBefore || idx+1 >= len(value) {
+				// can't end with an underscore
+				return errInvalidUnderscoreHex
+			}
+		}
+		hasBefore = isHexDigit(r)
 	}
 	return nil
 }
@@ -488,8 +506,3 @@ func parseToml(flow []token) *Tree {
 	parser.run()
 	return result
 }
-
-func init() {
-	numberUnderscoreInvalidRegexp = regexp.MustCompile(`([^\d]_|_[^\d])|_$|^_`)
-	hexNumberUnderscoreInvalidRegexp = regexp.MustCompile(`(^0x_)|([^\da-f]_|_[^\da-f])|_$|^_`)
-}