Regular Expressions

1A pattern describes a family of strings

A pattern is not one exact string. It names a shape: "one or more digits", "a word after equals". `re` is the standard-library module that applies those shapes to text.

Write patterns as raw strings so a backslash stays a backslash in the pattern, not a Python escape.

Figure. The pattern r"lab" is a shape that fits the word lab inside the lab notes. re.search applies that shape to the text.

Literal search

import re
print(re.search(r"lab", "the lab notes"))

2search finds one; findall lists them

`re.search(pattern, text)` returns a match object for the first place the pattern fits, or None. `match.group()` is the matched text.

`re.findall(pattern, text)` returns a list of all non-overlapping matches. Use search when you need "is it there"; findall when you need every hit.

Figure. In temp=42C pressure=7, search on digit-plus returns the first run 42. findall returns both runs: 42 and 7.

Two calls

1. searchFirst match or None. Then .group() for the text.
2. findallA list of every match, in order.

First digit run, then all

import re
text = "temp=42C pressure=7"
m = re.search(r"\d+", text)
print(m.group())
print(re.findall(r"\d+", text))

3A few tokens you will actually type

Digit-plus matches a run of digits. Word characters are letters, digits and underscore. A dot matches one character (except a newline). Square brackets name a choice, such as a vowel class.

That is enough for labelled lab lines. Do not collect a catalogue of lookarounds and flags here — read the `re` docs when a later lab needs one extra token.

No diagram — the idea is carried by the prose, code block or coding lab.

4Lab: pull digits from a labelled line

Search a sensor line for the first run of digits and print it.

Figure. In temp=42C, re.search for a run of digits matches 42. .group() is that matched text.

Takeaway

1. Hold thisSearch a sensor line for the first run of digits and print it.

Coding lab. Find the reading runs in the app, with checks on your output.