regex template

---

Compiled regular expressions with auto-escaped interpolations using Python 3.14's t-strings.

This only supports Python 3.14 (which is not yet released) because it relies on t-strings.

The problem: escaping regular expressions

Have you ever tried to use user input or variables in a regular expression and run into escaping issues?

For example, if you want to match a file extension that's stored in a variable:

>>> import re
>>> extension = ".txt"
>>> pattern = re.compile(rf"^.*{extension}$")
>>> text = "filetxt"
>>> if pattern.search(text):
...     print(f"{text} matched")
... else:
...     print(f"{text} did not match")
...
filetxt matched

Special regular expression characters like ., *, +, and ? need to be properly escaped when used in regular expressions.

We can use the re.escape function to manually escape each replacement field:

>>> import re
>>> extension = ".txt"
>>> pattern = re.compile(rf"^.*{re.escape(extension)}$")
>>> text = "filetxt"
>>> if pattern.search(text):
...     print(f"{text} matched")
... else:
...     print(f"{text} did not match")
...
filetxt did not match

This is tedious, especially with multiple f-string replacement fields.

The solution: auto-escaping with t-strings

The regex_template.compile function automatically escapes interpolated variables when using t-strings, while leaving the main pattern unescaped:

>>> import regex_template as ret
>>> extension = ".txt"
>>> pattern = ret.compile(rt"^.*{extension}$")
>>> text = "filetxt"
>>> if pattern.search(text):
...     print(f"{text} matched")
... else:
...     print(f"{text} did not match")
...
filetxt did not match

Replacement fields ({...}) are automatically escaped.

Note that regex_template.compile only accepts t-strings.

Safe interpolation

If you need to ensure specific replacement fields that are not escaped, use the :safe format specifier:

>>> import regex_template as ret
>>> part = "[^/]+"
>>> pattern = ret.compile(rt"/home/({part:safe})/Documents")
>>> text = "/home/trey/Documents"
>>> if match := pattern.search(text):
...     print(f"Matched Documents for user {match[1]}")
...
Matched Documents for user trey

Format specifiers

All standard Python format specifiers work normally and are applied before escaping:

>>> import regex_template as ret
>>> tracks = [(1, "Gloria"), (2, "Redondo Beach")]
>>> filename = "01 Gloria.mp3"
>>> for n, name in tracks:
...     pattern = ret.compile(rt"{n:02d}\ {name}\.mp3")
...     if pattern.fullmatch(filename):
...         print(f"Track {n} found!")
...
Track 1 found!

Verbose mode

By default, regex_template.compile enables verbose mode (re.VERBOSE) to encourage the use of more readable regular expressions:

import regex_template as ret

username = "trey"
hostname = "farnsworth"

# SSH log entry pattern
pattern = ret.compile(rt"""
    ^
    (\w{{3}} \s+ \d{{1,2}}) \s+         # Month and day ("Jan 1")
    (\d{{2}} : \d{{2}} : \d{{2}}) \s+   # Time ("14:23:45")
    {hostname} \s+                      # Server hostname (auto-escaped)
    sshd \[\d+\] : \s+                  # sshd process
    Accepted \s+ \w+ \s+                # Authentication method
    for \s+ {username} \s+              # Username (auto-escaped)
    from \s+ ([\d.]+) \s+               # IP address
    port \s+ \d+                        # Port number
""")

with open("sshd.log") as log_file:
    for line in log_file:
        if match := pattern.search(line):
            print("Login from IP {match[1]}")

You can set verbose=False to disable this:

pattern = ret.compile(
    rt"^(\w+ \d+ \d+:\d+:\d+) {hostname} .* for {username} from ([\d.]+)",
    verbose=False,
)

Installation

You can install regex-template with pip (you'll need to be on Python 3.14):

pip install regex-template

Or if you have uv installed and you'd like to play with it right now (Python 3.14 will be auto-installed):

uvx --with regex-template python

You can then import regex_template like this:

import regex_template as ret

Testing

This project uses hatch.

To run the tests:

hatch test

To see code coverage:

hatch test --cover
hatch run cov-html
open htmlcov/index.html

License

regex-template is distributed under the terms of the MIT license.