TopolearnPicking the right function is the difference between a working validator and one that accepts garbage.
A regular expression is a small pattern language for text. The three basic functions differ only in where the pattern must match:
re.search(pattern, text)finds the first match anywhere in the text.re.match(pattern, text)only matches at the start of the text.re.fullmatch(pattern, text)requires the pattern to match the entire text. Use it for validation.
Each returns a Match object on success or None on failure, so the usual idiom is:
if m := re.search(r"\d+", text):
print(m.group())m.group() (or m[0]) is the matched text, and m.start(), m.end() and m.span() give its position.
Write patterns as raw strings: r"\d+". In a normal string literal, Python processes backslashes first, so "\b" would be a backspace character instead of the regex word boundary. Raw strings pass backslashes through to the regex engine untouched.
Plain characters match themselves; the special characters . ^ $ * + ? { } [ ] \ | ( ) have meanings covered in the next lessons. Many tasks don't need regex at all: "abc" in text, text.startswith(...) and str.split are simpler and faster for fixed strings.
Example
import re
text = "Order 1234 shipped on 2025-03-01"
m = re.search(r"\d+", text)
print(m.group(), m.span()) # 1234 (6, 10)
print(re.match(r"\d+", text)) # None: text starts with 'O'
print(re.match(r"Order", text) is not None) # True
for candidate in ["2025", "2025a", "x2025"]:
print(candidate, bool(re.fullmatch(r"\d{4}", candidate)))
# 2025 True
# 2025a False
# x2025 False
print("\b" == "\x08", r"\b" == "\\b") # True TrueRemember
searchanywhere,matchat the start,fullmatchthe whole string- They return a Match object or None; test the result before using it
- Always write patterns as raw strings: r'...'
Watch out for
- Using
re.matchfor validation; it accepts any string that merely begins with a valid value.