This regexp:

/(?<=^)/

Works on 1.7.13, but not 1.7.15:

SyntaxError: (RegexpError) invalid pattern in look-behind

Specific versions:

• jruby 1.7.13 (1.9.3p392) 2014-08-28 655889c on Java HotSpot(TM) 64-Bit Server VM 1.7.0_60-b19 [linux-amd64]
• jruby 1.7.15 (1.9.3p392) 2014-09-20 82b5cc3 on Java HotSpot(TM) 64-Bit Server VM 1.7.0_60-b19 +jit [linux-amd64]
• It also works on MRI 1.9.3-p547.