Change using methods for matchers to be able to monkey patch them

Hi!

Because we need the possibility to monkey patch the slave matcher, I created this pull request. It's just using methods instead of the constants directly, so we are able to change them. 

e.g. we need to select using unions which needs a parentheses as first character
```
(SELECT * FROM ... )  UNION ( SELECT ... ) 
```

so we are able to monkey patch it to:
```
/^\(?select\s/i
```

Best regards,
Martin