Parse Expressions

Parse expressions are a simplified form of regular expressions. The actual regular expression is hidden behind the type name / hint.

Parse expressions are used in step definitions as a simplified alternative to regular expressions. They are used for parameters and type conversions (which are not supported for regular expression patterns).

# -- FILE: features/steps/example_steps.py
from behave import when

@when('we implement {number:d} tests')
def step_impl(context, number):  # -- NOTE: number is converted into integer
    assert number > 1 or number == 0
    context.tests_count = number

The following tables provide a overview of the parse expressions syntax. See also Python regular expressions description in the Python re module.

Type Characters Matched Output
l Letters (ASCII) str
w Letters, numbers and underscore str
W Not letters, numbers and underscore str
s Whitespace str
S Non-whitespace str
d Digits (effectively integer numbers) int
D Non-digit str
n Numbers with thousands separators (, or .) int
% Percentage (converted to value/100.0) float
f Fixed-point numbers float
F Decimal numbers Decimal
e Floating-point numbers with exponent e.g. 1.1e-10, NAN (all case insensitive) float
g General number format (either d, f or e) float
b Binary numbers int
o Octal numbers int
x Hexadecimal numbers (lower and upper case) int
ti ISO 8601 format date/time e.g. 1972-01-20T10:21:36Z (“T” and “Z” optional) datetime
te RFC2822 e-mail format date/time e.g. Mon, 20 Jan 1972 10:21:36 +1000 datetime
tg Global (day/month) format date/time e.g. 20/1/1972 10:21:36 AM +1:00 datetime
ta US (month/day) format date/time e.g. 1/20/1972 10:21:36 PM +10:30 datetime
tc ctime() format date/time e.g. Sun Sep 16 01:03:52 1973 datetime
th HTTP log format date/time e.g. 21/Nov/2011:00:07:11 +0000 datetime
ts Linux system log format date/time e.g. Nov 9 03:37:44 datetime
tt Time e.g. 10:21:36 PM -5:30 time

If parse_type module is used, the cardinality of a type can be specified, too (by using the CardinalityField support):

Cardinality Description
? Pattern with cardinality 0..1: optional part (question mark).
* Pattern with cardinality zero or more, 0.. (asterisk).
+ Pattern with cardinality one or more, 1.. (plus sign).