all: replace [0-9] with \d in regexps

1. replace [0-9] with \d in regexps
2. replace [a-zA-Z0-9_] with \w in regexps

Change-Id: I9e260538252a0c1071e76aeb1c5f885c6843a431
GitHub-Last-Rev: 286e1a4619c4bdda7f461afbd6d30b9f312c0486
GitHub-Pull-Request: golang/go#54874
Reviewed-on: https://go-review.googlesource.com/c/go/+/428435


Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>