Alphabetical index
- abs
 - ancestors
 - api
 - boolean
 - byte
 - c for strings, for booleans
 - cap_first
 - capitalize
 - ceiling
 - children
 - chop_linebreak
 - chunk
 - contains
 - date for dates, for strings
 - date_if_unknown
 - datetime for dates, for strings
 - datetime_if_unknown
 - double
 - ends_with
 - ensure_ends_with
 - ensure_starts_with
 - eval
 - first
 - floor
 - groups
 - float
 - has_api
 - has_content
 - html
 - index_of
 - int
 - interpret
 - is_infinite
 - is_nan
 - is_type
 - iso, iso_...
 - j_string
 - join
 - js_string
 - keep_after
 - keep_after_last
 - keep_before
 - keep_before_last
 - keys
 - last
 - last_index_of
 - left_pad
 - length
 - long
 - lower_abc
 - lower_case
 - matches
 - namespace
 - new
 - node_namespace
 - node_name
 - node_type
 - number
 - number_to_date, number_to_datetime, number_to_time
 - parent
 - replace
 - remove_beginning
 - remove_ending
 - reverse
 - right_pad
 - round
 - root
 - rtf
 - short
 - size
 - sort
 - seq_contains
 - seq_index_of
 - seq_last_index_of
 - sort_by
 - split
 - starts_with
 - string: for strings, for numbers, for booleans, for date/time/date-time
 - substring (deprecated)
 - time for date/time/date-time, for strings
 - time_if_unknown
 - trim
 - uncap_first
 - upper_abc
 - upper_case
 - url
 - values
 - word_list
 - xhtml
 - xml
 
See the built-ins filtered by left-hand-value type here.
If you don't find a built-in here that you have seen in a working template, probably you will find it here: Deprecated FTL constructs