fix(for): use IE compatible regex in v-for regex

[posva]

Closes #7946

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[posva]

Credit to @schoonc for pointing out the fix

[cleptric]

Is there something left, I could help with, to get this merged?

[cyberzed]

@sodatea is there any plans for merging this one or it's left in the dark alone forever :)

[ffMathy]

Code looks good.

[cleptric]

How can this take 6 months to get a fix for a breaking change, in a supported browser, getting merged? Does vue not care about IE10 anymore?

[TheWebTech]

I'm here to bump this PR again as @cleptric stated, this is a simple fix that fixes support for a major feature of Vue for IE.