Function: replace

--Usage--
> replace(some_string, match: regular expression, replace: replacement, in_context: regular expression)

Replace all matches of a regular expression with a replacement value.
The replacement can either be a string or a function.
* The string can contain backreference @"\\0"@, @"\\1"@, etc. refering to the components captured by the parentheses in the regular expression.
* When the replacement is a function, the variable @input@ contains the whole match and @_1@, @_2@, etc. contains the components.

Optionally a context can be given. The replacement is only performed if the string where the match is represented as <tt>&lt;match></tt> also matches the context.

When the @replace@ is used many times with the same @match@ or @in_context@ values it is more efficient to declare these as default arguments:
> my_replace := replace@(match: "something", replace: "something else")
> my_replace("input") # called many times
This way the regular expression is compiled only once.

--Parameters--
! Parameter	Type			Description
| @input@	[[type:string]]		String to replace in.
| @match@	[[type:regex]]		Regular expression to match.
| @replace@	[[type:string]] or [[type:function]]
		 			Replacement
| @in_context@	[[type:regex]] (optional)	Context to match

--Examples--
> replace(match: "a", replace: "e", "banana")  ==  "benene"
> replace(match: "a", replace: "e", in_context: "<match>n", "banana")  ==  "benena"
> replace(match: "(a|b)x", replace: "[\\0,\\1]", "axabxc")  ==  "[ax,a]a[bx,b]c"
> replace(match: "[ab]", replace: to_upper, "banana")  ==  "BAnAnA"
> replace(match: "([0-9])[*]([0-9])", replace: { _1 * _2 }, "2*2+3*3")  ==  "4+9"
>
> f := replace@(match: "xx+", replace: "A")
> f("xyzxxyyzz") == "xyzAAyyzz"

--See also--
| [[fun:filter_text]]	Keep only the text matching a regular expression.
