Regex, split and Python

Question

Can anybody help me to understand what those lines are doing ?

VAR_TOKEN_START = '{{'
VAR_TOKEN_END = '}}'
BLOCK_TOKEN_START = '{%'
BLOCK_TOKEN_END = '%}'
TOK_REGEX = re.compile(r"(%s.*?%s|%s.*?%s)" % (
    VAR_TOKEN_START,
    VAR_TOKEN_END,
    BLOCK_TOKEN_START,
    BLOCK_TOKEN_END
))

TOK_REGEX.split('{% each vars %}<i>{{it}}</i>{% endeach %}')

I don't understand the % on the regex expression. And why we split on TOK_REGEX variable expression.

Did you take a look at docs.python.org/2/library/re.html#re.split ? — Gnijuohz
– Gnijuohz, Commented Mar 23, 2014 at 22:25

user2357112 · Accepted Answer · 2014-03-23 22:43:32Z

1

This part:

TOK_REGEX = re.compile(r"(%s.*?%s|%s.*?%s)" % (
    VAR_TOKEN_START,
    VAR_TOKEN_END,
    BLOCK_TOKEN_START,
    BLOCK_TOKEN_END
))

uses string formatting to build a regex in a more understandable manner than just a jumble of characters. The % operator replaces each %s with the contents of the corresponding string in the following tuple. This allows the author of the code to give meaningful names to the {{, }}, {%, and %} parts of the regex.

The split call:

TOK_REGEX.split('{% each vars %}<i>{{it}}</i>{% endeach %}')

equivalent to the re.split function with the compiled pattern, finds all occurrences of text matching the regex in the argument string and returns a list of the parts divided by the matches - except that since the regex was in a capturing group (the parentheses in the regex string), the regex matches are also included in the list.

edited Mar 23, 2014 at 22:43

answered Mar 23, 2014 at 22:25

user2357112

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6 Comments

Mag-Stellon Over a year ago

The exact code is on this website : alexmic.net/building-a-template-engine

gat Over a year ago

Shouldn't this be a comment than an answer?

user2357112 Over a year ago

@gat: It answers the question, doesn't it?

user2357112 Over a year ago

@user3453437: Edit the proper code into the question.

Mag-Stellon Over a year ago

Split returns all words whithout the first expression given which is used as a separator ... But on the example, it's return an array with all word are well-separated ... I don't understand why.

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