internal/encoding/text: replace use of regular expression in decoding

Improve performance by replacing use of regular expressions with direct
parsing code.

Compared to latest version:

name                                     old time/op    new time/op    delta
Text/Unmarshal/google_message1_proto2-4    21.8µs ± 5%    14.0µs ± 9%  -35.69%  (p=0.000 n=10+9)
Text/Unmarshal/google_message1_proto3-4    19.6µs ± 4%    13.8µs ±10%  -29.47%  (p=0.000 n=10+10)
Text/Unmarshal/google_message2-4           13.4ms ± 4%     4.9ms ± 4%  -63.44%  (p=0.000 n=10+10)
Text/Marshal/google_message1_proto2-4      13.8µs ± 2%    14.1µs ± 4%   +2.42%  (p=0.011 n=9+10)
Text/Marshal/google_message1_proto3-4      11.6µs ± 2%    11.8µs ± 8%     ~     (p=0.573 n=8+10)
Text/Marshal/google_message2-4             8.01ms ±48%    5.97ms ± 5%  -25.44%  (p=0.000 n=10+10)

name                                     old alloc/op   new alloc/op   delta
Text/Unmarshal/google_message1_proto2-4    13.0kB ± 0%    12.6kB ± 0%   -3.40%  (p=0.000 n=10+10)
Text/Unmarshal/google_message1_proto3-4    13.0kB ± 0%    12.5kB ± 0%   -3.50%  (p=0.000 n=10+10)
Text/Unmarshal/google_message2-4           5.67MB ± 0%    5.50MB ± 0%   -3.13%  (p=0.000 n=10+10)
Text/Marshal/google_message1_proto2-4      12.0kB ± 0%    12.1kB ± 0%   +0.02%  (p=0.000 n=10+10)
Text/Marshal/google_message1_proto3-4      11.7kB ± 0%    11.7kB ± 0%   +0.01%  (p=0.000 n=10+10)
Text/Marshal/google_message2-4             5.68MB ± 0%    5.68MB ± 0%   +0.01%  (p=0.000 n=10+10)

name                                     old allocs/op  new allocs/op  delta
Text/Unmarshal/google_message1_proto2-4       142 ± 0%       142 ± 0%     ~     (all equal)
Text/Unmarshal/google_message1_proto3-4       156 ± 0%       156 ± 0%     ~     (all equal)
Text/Unmarshal/google_message2-4            70.1k ± 0%     65.4k ± 0%   -6.76%  (p=0.000 n=10+10)
Text/Marshal/google_message1_proto2-4        91.0 ± 0%      91.0 ± 0%     ~     (all equal)
Text/Marshal/google_message1_proto3-4        80.0 ± 0%      80.0 ± 0%     ~     (all equal)
Text/Marshal/google_message2-4              36.4k ± 0%     36.4k ± 0%     ~     (all equal)

Change-Id: Ia5d3c16e9e33961aae03bac0d53fcfc5b1943d2a
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/173360
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/internal/encoding/text/encode.go b/internal/encoding/text/encode.go
index 982037b..1f49761 100644
--- a/internal/encoding/text/encode.go
+++ b/internal/encoding/text/encode.go
@@ -6,6 +6,7 @@
 
 import (
 	"bytes"
+	"regexp"
 	"strings"
 
 	"google.golang.org/protobuf/internal/detrand"
@@ -142,6 +143,12 @@
 	return nil
 }
 
+// This expression is more liberal than ConsumeAnyTypeUrl in C++.
+// However, the C++ parser does not handle many legal URL strings.
+// The Go implementation is more liberal to be backwards compatible with
+// the historical Go implementation which was overly liberal (and buggy).
+var urlRegexp = regexp.MustCompile(`^[-_a-zA-Z0-9]+([./][-_a-zA-Z0-9]+)*`)
+
 func (p *encoder) marshalKey(v Value) error {
 	switch v.Type() {
 	case String: