--- title: "regexp_extract" slug: "dev-database-reference-sql-functions-string-regexp-extract" updated: 2026-10-30T18:18:56Z published: 2026-10-30T18:18:56Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-string-regexp-extract" --- > ## Documentation Index > Fetch the complete documentation index at: https://docs.vastdata.com/llms.txt > Use this file to discover all available pages before exploring further. # regexp_extract *Applies to VAST Cluster 5.5.* *[Functions](/develop/docs/dev-database-reference-sql-functions-index) › [Regular expressions](/develop/docs/dev-database-reference-sql-functions-string-regular-expressions) › Regular expressions.* `regexp_extract(s, pattern)` returns the first part of `s` that the regular expression `pattern` matches, or an empty string if nothing matches. With a group number it returns that group of the match; with a list of names it returns a `STRUCT` with one field per group. ## Syntax ``` regexp_extract(s, pattern) regexp_extract(s, pattern, group) regexp_extract(s, pattern, group, options) regexp_extract(s, pattern, names) regexp_extract(s, pattern, names, options) ``` | Argument | Type | Description | | --- | --- | --- | | `s` | `VARCHAR` | The string to search. | | `pattern` | `VARCHAR` | The regular expression ([syntax](/develop/docs/dev-database-reference-sql-functions-string-regexp-syntax)). A constant or a column. | | `group` | `INTEGER`, a constant from 0 to 9 | Optional. `0` (the default) returns the whole match; `1` to `9` the group with that number. | | `names` | `VARCHAR[]`, a constant | Instead of `group`: one field name for each group, in order. | | `options` | `VARCHAR` | Optional, a constant. Flags such as `'i'` (ignore case); see [options](/develop/docs/dev-database-reference-sql-functions-string-regexp-syntax#what-it-does). | **Returns:** `VARCHAR`; with `names`, a `STRUCT` with one `VARCHAR` field per name. ## Set up the examples The examples connect with the ADBC driver, as in [VAST Query Engine (ADBC)](/develop/docs/dev-database-integrations-adbc), and work in a schema called `docs_sql_fn_string_regexp_extract`. Set `QE_ENDPOINT`, `AWS_ACCESS_KEY_ID`, `AWS_SECRET_ACCESS_KEY` and `VASTDB_BUCKET` ([SQL reference](/develop/docs/dev-database-reference-sql-index#example-connect-and-use-a-transaction)), and run the blocks in order in one Python session. Below each block is the output you should see. This block connects, defines the `run()` and `plan()` helpers, and creates a table of four file paths. Path 3 has no date, and the last row has no values: ``` import os import re import adbc_driver_manager import adbc_driver_manager.dbapi as dbapi import adbc_driver_vastdb BUCKET = os.environ["VASTDB_BUCKET"] SCHEMA = f'"{BUCKET}/docs_sql_fn_string_regexp_extract"' conn = dbapi.connect( driver=adbc_driver_vastdb.get_driver_path(), db_kwargs={ "vast.db.endpoint": os.environ["QE_ENDPOINT"], "vast.db.access_key": os.environ["AWS_ACCESS_KEY_ID"], "vast.db.secret_key": os.environ["AWS_SECRET_ACCESS_KEY"], }, autocommit=True, ) def server_message(e): """The server's message in a driver error, without the request ID or the echoed statement.""" text = re.sub(r"\[req=0x[0-9a-f]+\]: ", "", str(e)) found = (re.search(r'message: "(.*?)", request_id', text, re.S) or re.search(r"description: (.*)\)", text, re.S)) message = found.group(1) if found else text.strip().splitlines()[-1] message = re.sub(r"^Plan query: '.*?' failed with: ", "", message, flags=re.S) message = re.split(r"\\n|\n", message)[0].replace('\\"', '"') return message.replace(BUCKET, "") def run(sql): """Run one statement. Print the rows it returns, or the error.""" try: with conn.cursor() as cur: cur.execute(sql) rows = cur.fetchall() if cur.description else [] except (adbc_driver_manager.Error, OSError) as e: print(f"{type(e).__name__}: {server_message(e)}") else: print(rows) def plan(sql): """Print the plan without its ~N Rows lines: the table's row count, which can still be 0 just after an insert.""" with conn.cursor() as cur: cur.execute("EXPLAIN " + sql) steps = cur.fetchall()[0][1] print("\n".join(line for line in steps.splitlines() if "Rows" not in line)) TABLE = f"{SCHEMA}.files" run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}") run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, path VARCHAR)") run(f"INSERT INTO {TABLE} VALUES " "(1, '/data/2026-10-16/app.log'), " "(2, '/data/2026-10-17/db.csv'), " "(3, '/tmp/README'), " "(4, NULL)") ``` ``` [] [] [(4,)] ``` ## What it does `regexp_extract` finds the **first** match of `pattern` in `s`, then returns: | Form | Returns | Example | Result | | --- | --- | --- | --- | | no `group` (or `0`) | the whole match | `regexp_extract('a1b22', '\d+')` | `'1'` | | `group` 1 to 9 | that group of the match | `regexp_extract('abc', '(a)(b)', 2)` | `'b'` | | `names` | a `STRUCT` of the groups, named in order | `regexp_extract('2026-10', '(\d+)-(\d+)', ['y', 'm'])` | `{'y': '2026', 'm': '10'}` | Without a match, the result is an empty string, not `NULL`; with `names`, every field is an empty string. A group that doesn't take part in the match, such as `(x)?` in `'(x)?b'`, also gives `''`. Groups are numbered by their opening parenthesis; `(?:...)` doesn't count, and a named group `(?P<n>...)` counts like any other. Read a field of the `STRUCT` with `.name` or `['name']`. ## NULLs and edge cases | Input | Result | | --- | --- | | A `NULL` string or pattern | `NULL` | | A `NULL` group, or a `NULL` name list | `''` | | A `NULL` options string | Error: `Regex options field must not be NULL` | | `regexp_extract('abc', 'x')` (no match) | `''` | | `regexp_extract('abc', '(a)', 2)` (the pattern has one group) | `''` | | `regexp_extract('abc', '(a)', 10)` or `-1` | Error: `Group index must be between 0 and 9!` | | `regexp_extract('e\u0301x', '^.')` | `'e'` (a `.` is one code point) | | `regexp_extract('abc', '(')` (an invalid pattern) | `''`, with no error | | `regexp_extract('2026-10', '(\d+)', ['y', 'm'])` (more names than groups) | Error: `Not enough group names in regexp_extract` | | `regexp_extract('2026-10', '(\d+)-(\d+)', ['y'])` (fewer names) | `{'y': '2026'}` | The same cases, as the cluster returns them. They use the table and helpers from [Set up the examples](/develop/docs/dev-database-reference-sql-functions-string-regexp-extract#set-up-the-examples): ``` run("SELECT regexp_extract(NULL, 'a'), regexp_extract('abc', NULL), regexp_extract('abc', '(a)', NULL), " "regexp_extract('abc', 'x'), regexp_extract('abc', '(a)', 2)") run("SELECT regexp_extract('abc', '(b)', 1, NULL)") run("SELECT regexp_extract('abc', '(a)', 10)") run("SELECT regexp_extract('e\u0301x', '^.'), regexp_extract('abc', '(')") run("SELECT regexp_extract('2026-10', '(\\d+)', ['y', 'm'])") run("SELECT regexp_extract('2026-10', '(\\d+)-(\\d+)', ['y'])") ``` ``` [(None, None, '', '', '')] InternalError: Invalid Input Error: Regex options field must not be NULL InternalError: Invalid Input Error: Group index must be between 0 and 9! [('e', '')] InternalError: Binder Error: Not enough group names in regexp_extract [({'y': '2026'},)] ``` ## Examples The date in each path, its month (group 2), and the file extension. Path 3 has no date, so the first two are empty strings: ``` run(f"SELECT id, regexp_extract(path, '\\d{{4}}-\\d{{2}}-\\d{{2}}'), " f"regexp_extract(path, '(\\d{{4}})-(\\d{{2}})', 2), regexp_extract(path, '\\.(\\w+)$', 1) " f"FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, '2026-10-16', '10', 'log'), (2, '2026-10-17', '10', 'csv'), (3, '', '', ''), (4, None, None, None)] ``` Split the date into a `STRUCT` with named fields, and read one field: ``` run(f"SELECT id, regexp_extract(path, '(\\d+)-(\\d+)-(\\d+)', ['y', 'm', 'd']), " f"regexp_extract(path, '(\\d+)-(\\d+)-(\\d+)', ['y', 'm', 'd']).m FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, {'y': '2026', 'm': '10', 'd': '16'}, '10'), (2, {'y': '2026', 'm': '10', 'd': '17'}, '10'), (3, {'y': '', 'm': '', 'd': ''}, ''), (4, None, None)] ``` Ignore case with option `'i'`: ``` run(f"SELECT id, regexp_extract(path, '/([a-z]+)$', 1), regexp_extract(path, '/([a-z]+)$', 1, 'i') " f"FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, '', ''), (2, '', ''), (3, '', 'README'), (4, None, None)] ``` ## How it runs in VAST The Query Engine runs on the cluster's CNodes, and a query on a table starts with a scan in VAST DataBase (`VAST_SCAN`). `regexp_extract` is always evaluated in the Query Engine: - **In the `SELECT` list,** the scan reads the column and the Query Engine extracts from each value it returns. - **In `WHERE`,** the scan sends the column for every row, and a `FILTER` step computes `regexp_extract()` and keeps the rows that match. ``` plan(f"SELECT id FROM {TABLE} WHERE regexp_extract(path, '\\d{{4}}') = '2026'") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #1 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ (regexp_extract(path, ' │ │ \d{4}') = '2026') │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ path │ │ id │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Extract at load time.** If you filter on an extracted part often, such as the date of a path, extract it when you write the data and store it in its own column. A comparison of that column with a constant is pushed down to VAST DataBase. - **Test with a pattern, not an extraction.** To keep the rows that contain a match, `regexp_matches(path, '\d{4}')` is rewritten into cheaper tests for simple patterns ([regexp_matches](/develop/docs/dev-database-reference-sql-functions-string-regexp-matches#how-it-runs-in-vast)); `regexp_extract(path, ...) <> ''` isn't. - **Extract several groups in one call.** One call with a name list returns every group of one match. ## Limitations and errors - **Group numbers 0 to 9 only,** and a constant: a group from a column fails with `Group specification field must be a constant!`, and so does a name list from a column. - **Name a group with the list form.** A string such as `'n'` as the third argument fails with `Could not choose a best candidate function`; write `['n']`. - **Name lists have rules.** Empty: `regexp_extract requires non-empty lists of capture names`. A repeated name: `Duplicate group name "y" in regexp_extract`. A `NULL` name: `NULL group name in regexp_extract`. A typed `NULL` list such as `NULL::VARCHAR[]` fails with an internal error; pass a list literal. - **An invalid constant pattern returns `''`** instead of failing. Check a pattern with [regexp_matches](/develop/docs/dev-database-reference-sql-functions-string-regexp-matches), which reports the error. - **No `regexp_substr`.** It fails with `Scalar Function with name regexp_substr does not exist!`; use `regexp_extract`. ``` run("SELECT regexp_extract('abc', '(?Pb)', 'n')") run("SELECT regexp_extract('abc', '(?Pb)', ['n'])") run("SELECT regexp_extract('2026-10', '(\\d+)-(\\d+)', ['y', 'y'])") run("SELECT regexp_matches('abc', '(')") run("SELECT regexp_substr('abc', 'b')") ``` ``` InternalError: Binder Error: Could not choose a best candidate function for the function call "regexp_extract(STRING_LITERAL, STRING_LITERAL, STRING_LITERAL)". In order to select one, please add explicit type casts. [({'n': 'b'},)] InternalError: Binder Error: Duplicate group name "y" in regexp_extract InternalError: Invalid Input Error: missing ): ( InternalError: Catalog Error: Scalar Function with name regexp_substr does not exist! ``` ## Similar functions | Function | Returns | Result for `'a,b,c'` | | --- | --- | --- | | `regexp_extract('a,b,c', '^[^,]*,([^,]*)', 1)` | one group of the first match | `'b'` | | [regexp_extract_all](/develop/docs/dev-database-reference-sql-functions-string-regexp-extract-all)`('a,b,c', '[^,]+')` | every match, as a list | `['a', 'b', 'c']` | | [split_part](/develop/docs/dev-database-reference-sql-functions-string-split-part)`('a,b,c', ',', 2)` | one field between literal separators | `'b'` | | [string_split](/develop/docs/dev-database-reference-sql-functions-string-string-split)`('a,b,c', ',')[2]` | a list of fields, then one element | `'b'` | | [regexp_matches](/develop/docs/dev-database-reference-sql-functions-string-regexp-matches)`('a,b,c', ',')` | whether there's a match | `true` | ``` run("SELECT regexp_extract('a,b,c', '^[^,]*,([^,]*)', 1), regexp_extract_all('a,b,c', '[^,]+'), " "split_part('a,b,c', ',', 2), string_split('a,b,c', ',')[2], regexp_matches('a,b,c', ',')") ``` ``` [('b', ['a', 'b', 'c'], 'b', 'b', True)] ``` Clean up: ``` run(f"DROP TABLE IF EXISTS {TABLE}") run(f"DROP SCHEMA IF EXISTS {SCHEMA}") conn.close() ``` ``` [] [] ``` ## See also - [Regular-expression syntax and options](/develop/docs/dev-database-reference-sql-functions-string-regexp-syntax). - [Regular expressions](/develop/docs/dev-database-reference-sql-functions-string-regular-expressions): all the regular-expression functions. - [Where filters run](/develop/docs/dev-database-reference-sql-select#where-filters-run) and [pushdowns](/develop/docs/dev-database-reference-pushdowns). - [EXPLAIN](/develop/docs/dev-database-reference-sql-explain): how to read a plan.