Greedy and Lazy Tags

A tag is marked with the symbols < and > and can be either of two types: named like <foo-name> or unnamed like <...>.

Both kinds of tags can match anything but there is a small difference.

Because the named tags are used to capture a given string and paste it later, it is assumed that a named tag is intended to match a small string, therefore the regex used is non-greedy or lazy (.*?).

The usage of unnamed tags is more diffuse: they can be used to ignore small portions or large multiline ones.

The heuristic is that the unnamed tags are non-greedy by default but the unamed tags at the end of a line are greedy (.*).

Here are some examples of the implications of this difference.

Consider the following string.

>>> a = '''x 1 oneline
... x 2 twoline
... x 3 fooline
... x 4 barline'''

If we are interested in only the last two numbers we could write something like this:

>>> print(a)
<...>
x <foo> fooline
x <bar> barline

>>> print("""foo: '<foo>'\nbar: '<bar>'""")       # byexample: +paste
foo: '3'
bar: '4'

This works because the unnamed tag is at the end of the line and therefore its regex is greedy.

If this wasn’t the case, the named tag below will probably be forced to capture more strings than intended.

Here is the same example but instead of using an unnamed tag, we use a named tag to force it to be non-greedy. See what happens with the foo and bar captures:

>>> print(a)
<ignore-me>
x <foo> fooline
x <bar> barline

>>> print("""foo: '<foo>'\nbar: '<bar>'""")       # byexample: +paste
foo: '2 twoline
x 3'
bar: '4'

Consider now another example. It works because the unnamed tags are non-greedy except on the end (in this example we want to capture Joe’s token)

>>> print('{user=john,attr=2,token=53,age=33;user=joe,attr=3,token=111,age=33;user=jane,attr=12,token=153,age=3}')
<...>user=joe,<...>,token=<token>,<...>

>>> print("""token: '<token>'""")       # byexample: +paste
token: '111'