You can do that directly in Twig now:
{% if 'World' starts with 'F' %}
{% endif %}
“Ends with” is also supported:
{% if 'Hello' ends with 'n' %}
{% endif %}
Other handy keywords also exist:
Complex string comparisons:
{% if phone matches '{^[\\d\\.]+$}' %} {% endif %}
(Note: double backslashes are converted to one backslash by twig)
String contains:
{{ 'cd' in 'abcde' }}
{{ 1 in [1, 2, 3] }}
See more information here: http://twig.sensiolabs.org/doc/templates.html#comparisons