regexp: add the missing is
regexp: add the missing is

Change-Id: I23264972329aa3414067cd0e0986b69bb39bbeb5
GitHub-Last-Rev: d1d668a3cbe852d9a06f03369e7e635232d85139
GitHub-Pull-Request: golang/go#50650
Reviewed-on: https://go-review.googlesource.com/c/go/+/378935
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Trust: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>

CAFxX/go,cloudflare/go,golang/go,golang/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go