## TL;DR
A regex expectation failure is a pattern-versus-data argument: extract the values that do not match, lay them beside the pattern, and decide whether the data format changed or the pattern is too narrow, for example missing a country prefix or a new separator. Test the revised pattern against both the failing values and a sample of previously passing values before deploying it.

```text
how to fix great expectations error failed regex match expect_column_values_to_match_regex failure
```

## Use this when
- The check or run fails with this symptom: values in the column fail the expect_column_values_to_match_regex pattern.
- You are working on a Great Expectations regex expectation failing on column values and the failure repeats on the same data or configuration.
- You need the failing rows, rule, or stage isolated before changing thresholds or code.

## Not for this skill when
- A type or null expectation failure, where the values exist but the rule type differs instead.
- The error text names a different tool, stage, or root cause. Match the verbatim error first and use that skill.

## Steps
1. Extract the non-matching values from the validation result and group them by shape. Verify: the non-matching value shapes are grouped
2. Write the current regex out and test it against representatives of each shape to see exactly where it rejects them. Verify: the rejection point per shape is pinpointed
3. Check the producing system for a format change, such as a new prefix, separator, or length. Verify: a format change is confirmed or ruled out
4. Fix the producer if the format change is accidental, or widen the regex deliberately if the new format is legitimate, keeping anchors and character classes strict. Verify: pattern and data agree by an explicit decision
5. Validate the whole column with the final regex and confirm previously passing values still pass. Verify: the full column passes with no regressions on old values

## Variant phrasings
### great expectations regex match failed expect_column_values_to_match_regex
The verbatim regex-expectation phrasing.
### regex expectation failing new value format
The format-change phrasing for evolved values.
### how to debug regex expectation unexpected values
The debugging phrasing for grouping non-matching shapes.

Compatibility: Great Expectations regex expectations such as expect_column_values_to_match_regex

## Why it happens
Regex expectations encode a format contract precisely, so any format evolution, even a helpful one like adding country codes, fails until the pattern catches up. Conversely, sloppy patterns pass formats the contract never intended, which is why both directions need testing.

## Edge cases / pitfalls
- Keep a small corpus of known-good and known-bad values as a test for the regex itself.
- Anchor patterns fully. Partial matches let malformed values slip through silently.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_TFdadyuVdSzwAje4HC0fwg