Allow underscore at start of placeholder names

[torohill]

Would it be possible to allow underscores at the start of placeholder names? Currently placeholders can only start with letters. PHP variables can start with underscores and I don't see any technical reason that this shouldn't be possible, but perhaps I'm missing something.

The following patch implements the change.

diff -r 00e7c8444f17 vendor/nikic/fast-route/src/RouteParser/Std.php
--- a/vendor/nikic/fast-route/src/RouteParser/Std.php   Wed May 18 09:12:21 2016 +1200
+++ b/vendor/nikic/fast-route/src/RouteParser/Std.php   Fri May 20 11:21:35 2016 +1200
@@ -13,7 +13,7 @@
class Std implements RouteParser {
    const VARIABLE_REGEX = <<<'REGEX'
\{
-    \s* ([a-zA-Z][a-zA-Z0-9_-]*) \s*
+    \s* ([a-zA-Z_][a-zA-Z0-9_-]*) \s*
    (?:
        : \s* ([^{}]*(?:\{(?-1)\}[^{}]*)*)
    )?
\}

[nikic]

Sounds reasonable, done: 30470b6