---
title: "Regular-expression syntax and options"
slug: "dev-database-reference-sql-functions-string-regexp-syntax"
updated: 2026-10-30T18:18:56Z
published: 2026-10-30T18:18:56Z
canonical: "docs.vastdata.com/dev-database-reference-sql-functions-string-regexp-syntax"
---

> ## 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.

# Regular-expression syntax and options

*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.* The regular-expression functions ([regexp_matches](/develop/docs/dev-database-reference-sql-functions-string-regexp-matches), [regexp_full_match](/develop/docs/dev-database-reference-sql-functions-string-regexp-full-match) and `~`, [regexp_extract](/develop/docs/dev-database-reference-sql-functions-string-regexp-extract), [regexp_extract_all](/develop/docs/dev-database-reference-sql-functions-string-regexp-extract-all), [regexp_replace](/develop/docs/dev-database-reference-sql-functions-string-regexp-replace) and the [split functions](/develop/docs/dev-database-reference-sql-functions-string-regexp-split-to-array)) share one pattern syntax and one options string. This page describes both, and the pattern features that aren't available.

## Syntax

A pattern and an options string are arguments of these functions:

```
regexp_matches(s, pattern)
regexp_matches(s, pattern, options)
regexp_full_match(s, pattern, options)
regexp_extract(s, pattern, group, options)
regexp_extract_all(s, pattern, group, options)
regexp_replace(s, pattern, replacement, options)
```

| Argument | Type | Description |
| --- | --- | --- |
| `pattern` | `VARCHAR` | The regular expression. A constant, or an expression such as a column. |
| `options` | `VARCHAR` | Optional. A constant string of one-letter flags, such as `'i'` or `'gi'`. |

## 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_syntax`. 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 five log lines. Line 4 has a line break in it, 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_syntax"'

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, "<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}.logs"
run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}")
run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, line VARCHAR)")
run(f"INSERT INTO {TABLE} VALUES "
    "(1, 'ERROR 2026-10-16 disk full'), "
    "(2, 'warn: Disk 93% full'), "
    "(3, 'caf\u00e9 cr\u00e8me'), "
    "(4, 'Line1\nLine2'), "
    "(5, NULL)")
```

```
[]
[]
[(5,)]
```

## What it does

**Writing a pattern.** In a SQL string literal a backslash is an ordinary character, so `&apos;\d+&apos;` in SQL is the pattern `\d+`. In a Python string, write it `&apos;\\d+&apos;`, as on this page.

| Pattern element | Meaning | Notes |
| --- | --- | --- |
| `.` | any one character (code point) | Not a line break, unless option `s` or `(?s)`. `e` with a combining accent (U+0301) is two characters, and so is a flag emoji. |
| `^`, `&#x24;` | start and end of the string | Not at line breaks unless `(?m)`. `&#x24;` doesn't match before a final line break: `&apos;a&apos;` + line break doesn't match `&apos;a&#x24;&apos;`. |
| `\A`, `\z`, `\b` | start of the string, end of the string, a word boundary |  |
| `[abc]`, `[^abc]`, `[a-z]` | a character set |  |
| `\d`, `\w`, `\s` | an ASCII digit, an ASCII word character, white space | `\d` doesn't match other digits such as `&apos;&#x663;&apos;`, `\w` doesn't match `&apos;&#xe9;&apos;`, and `\s` matches a tab but not a no-break space (U+00A0). |
| `[[:digit:]]`, `[[:alpha:]]` and other POSIX classes | ASCII classes | `[[:alpha:]]` doesn't match `&apos;&#xe9;&apos;`. An unknown class such as `[[:foo:]]` fails. |
| `\pL`, `\pN`, `\p{Greek}`, `\p{Han}`, `\p{Latin}` | Unicode letters, numbers and scripts | Use these for text outside ASCII. |
| `\x41`, `\x{1F600}` | a character by its code |  |
| `\Q...\E` | the text between is literal | `&apos;\Qa.b\E&apos;` matches `a.b` only. |
| `*`, `+`, `?`, `{n}`, `{n,m}` | repetition | `{n}` up to 1000. Lazy forms such as `*?` are accepted. |
| `(...)`, `(?:...)`, `(?P&lt;name&gt;...)` | a group, a group that doesn't capture, a named group |  |
| `a\\|b` | either |  |
| `(?i)`, `(?-i)`, `(?s)`, `(?m)` | inline flags: ignore case, stop ignoring case, `.` matches a line break, `^` and `&#x24;` at line breaks | An inline flag overrides the options string. |

The **options string** is a constant of one-letter flags:

| Option | Effect |
| --- | --- |
| `i` | Ignore case, including outside ASCII (`&apos;&#xc9;COLE&apos;` matches `&apos;&#xe9;cole&apos;`). One letter never matches two: `&apos;STRASSE&apos;` doesn't match `&apos;stra&#xdf;e&apos;`. |
| `c` | Case-sensitive (the default). With both `i` and `c`, the last one wins: `&apos;ic&apos;` is case-sensitive, `&apos;ci&apos;` isn't. |
| `s` | `.` also matches a line break. |
| `l` | The pattern is literal text, not a regular expression. Combine it with `i`: `&apos;li&apos;`. |
| `g` | Replace every match ([regexp_replace](/develop/docs/dev-database-reference-sql-functions-string-regexp-replace) only; other functions fail with `Option &apos;g&apos; (global replace) is only valid for regexp_replace`). |
| `m`, `n`, `p` | Accepted. They don't make `^` and `&#x24;` match at line breaks, or `.` match one; use `(?m)` or `s`. |
| `&apos;&apos;` | The default. |

Options are lower-case: `&apos;I&apos;`, `&apos;G&apos;`, `&apos;x&apos;` and any other letter fail with `Unrecognized Regex option`.

## NULLs and edge cases

| Input | Result |
| --- | --- |
| A `NULL` string or pattern | `NULL` |
| A `NULL` options string | Error: `Regex options field must not be NULL` (but [regexp_replace](/develop/docs/dev-database-reference-sql-functions-string-regexp-replace) returns `NULL`) |
| Options from a column | Error: `Regex options field must be a constant` |
| An empty pattern `&apos;&apos;` | Matches every string, the empty one too |
| `regexp_matches(&apos;ab&apos;, &apos;(?i)A(?-i)b&apos;)` | `true` |
| `regexp_matches(&apos;ABC&apos;, &apos;(?i)b&apos;, &apos;c&apos;)` (the inline flag wins) | `true` |
| `regexp_matches(&apos;ABC&apos;, &apos;b&apos;, &apos;ic&apos;)`, `regexp_matches(&apos;ABC&apos;, &apos;b&apos;, &apos;ci&apos;)` | `false`, `true` |
| `regexp_matches(&apos;a.c&apos;, &apos;a.c&apos;, &apos;l&apos;)`, `regexp_matches(&apos;abc&apos;, &apos;a.c&apos;, &apos;l&apos;)` | `true`, `false` |
| `regexp_matches(&apos;e\u0301&apos;, &apos;^.&#x24;&apos;)`, `regexp_matches(&apos;e\u0301&apos;, &apos;^..&#x24;&apos;)` | `false`, `true` |

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-syntax#set-up-the-examples):

```
run("SELECT regexp_matches(NULL, 'a'), regexp_matches('abc', NULL), regexp_matches('abc', ''), regexp_matches('', '')")
run("SELECT regexp_matches('abc', 'b', NULL)")
run(f"SELECT regexp_matches(line, 'a', CAST(id AS VARCHAR)) FROM {TABLE} LIMIT 1")
run("SELECT regexp_matches('ab', '(?i)A(?-i)b'), regexp_matches('ABC', '(?i)b', 'c'), "
    "regexp_matches('ABC', 'b', 'ic'), regexp_matches('ABC', 'b', 'ci')")
run("SELECT regexp_matches('a.c', 'a.c', 'l'), regexp_matches('abc', 'a.c', 'l'), "
    "regexp_matches('e\u0301', '^.$'), regexp_matches('e\u0301', '^..$')")
```

```
[(None, None, True, True)]
InternalError: Invalid Input Error: Regex options field must not be NULL
InternalError: Invalid Input Error: Regex options field must be a constant
[(True, True, False, True)]
[(True, False, False, True)]
```

## Examples

Character classes: `\d` for digits, `\w` and `\pL` for letters. `\w+&#x24;` doesn't match the last word of `caf&#xe9; cr&#xe8;me`, because `\w` is ASCII only; `\pL+&#x24;` does:

```
run(f"SELECT id, regexp_matches(line, '\\d{{4}}-\\d{{2}}-\\d{{2}}'), regexp_matches(line, '\\s\\w+$'), "
    f"regexp_matches(line, '\\s\\pL+$') FROM {TABLE} ORDER BY id LIMIT 10")
```

```
[(1, True, True, True), (2, False, True, True), (3, False, False, True), (4, False, True, False), (5, None, None, None)]
```

Options and inline flags: `&apos;disk&apos;` with `&apos;i&apos;` also matches `Disk`. In line 4, `^Line2` matches only with `(?m)`, and `1.L` (a dot for the line break) only with `&apos;s&apos;`:

```
run(f"SELECT id, regexp_matches(line, 'disk', 'i'), regexp_matches(line, '^Line2'), "
    f"regexp_matches(line, '(?m)^Line2'), regexp_matches(line, '1.L'), regexp_matches(line, '1.L', 's') "
    f"FROM {TABLE} ORDER BY id LIMIT 10")
```

```
[(1, True, False, False, False, False), (2, True, False, False, False, False), (3, False, False, False, False, False), (4, False, False, True, False, True), (5, None, None, None, None, None)]
```

## How it runs in VAST

An invalid constant pattern fails the query with `Invalid Input Error:` and the reason, such as `missing ): (`. A pattern from a column is checked for each row: one invalid value fails the query, with the reason but without the `Invalid Input Error:` prefix. [regexp_extract](/develop/docs/dev-database-reference-sql-functions-string-regexp-extract) and [regexp_replace](/develop/docs/dev-database-reference-sql-functions-string-regexp-replace) behave differently with an invalid constant pattern: see their pages.

Patterns are always evaluated in the Query Engine. Before it plans a query, the Query Engine rewrites some `regexp_matches` patterns into cheaper tests:

| Pattern in `WHERE regexp_matches(s, ...)` | Becomes | Runs in |
| --- | --- | --- |
| `&apos;^literal&#x24;&apos;` (the whole string, no special characters) | `s = &apos;literal&apos;` | VAST DataBase (pushed down; no `FILTER` step) |
| `&apos;^literal&apos;` | `prefix(s, &apos;literal&apos;)` | a `FILTER` step in the Query Engine |
| `&apos;literal&#x24;&apos;` | `suffix(s, &apos;literal&apos;)` | a `FILTER` step |
| `&apos;literal&apos;` | `contains(s, &apos;literal&apos;)` | a `FILTER` step |
| anything else, such as `&apos;^(a\\|b)&apos;` or `&apos;\d+&apos;` | `regexp_matches(...)` | a `FILTER` step |

The plans of a whole-string pattern, a prefix and a general pattern:

```
plan(f"SELECT id FROM {TABLE} WHERE regexp_matches(line, '^Hello World$')")
plan(f"SELECT id FROM {TABLE} WHERE regexp_matches(line, '^ERROR')")
plan(f"SELECT id FROM {TABLE} WHERE regexp_matches(line, '\\d+')")
```

```
┌───────────────────────────┐
│         VAST_SCAN         │
│    ────────────────────   │
│    Function: VAST_SCAN    │
│      Projections: id      │
│                           │
└───────────────────────────┘
┌───────────────────────────┐
│         PROJECTION        │
│    ────────────────────   │
│             #1            │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│           FILTER          │
│    ────────────────────   │
│   prefix(line, 'ERROR')   │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│         VAST_SCAN         │
│    ────────────────────   │
│    Function: VAST_SCAN    │
│                           │
│        Projections:       │
│            line           │
│             id            │
│                           │
└───────────────────────────┘
┌───────────────────────────┐
│         PROJECTION        │
│    ────────────────────   │
│             #1            │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│           FILTER          │
│    ────────────────────   │
│regexp_matches(line, '\d+')│
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│         VAST_SCAN         │
│    ────────────────────   │
│    Function: VAST_SCAN    │
│                           │
│        Projections:       │
│            line           │
│             id            │
│                           │
└───────────────────────────┘
```

## Make it faster

- **Prefer a literal or anchored pattern.** `&apos;^ERROR&apos;` runs as `prefix()` and `&apos;disk&apos;` as `contains()`; a whole-string pattern is pushed down as an equality.
- **Use a range for a prefix.** `line &gt;= &apos;ERROR&apos; AND line &lt; &apos;ERROS&apos;` is pushed down; see [LIKE](/develop/docs/dev-database-reference-sql-functions-string-like#make-it-faster).
- **Use a constant pattern.** A mistake in it fails the query whatever the data, instead of only when a bad value turns up.
- **Extract at load time.** If you filter on a part of a string often, store it in its own column with [regexp_extract](/develop/docs/dev-database-reference-sql-functions-string-regexp-extract) when you write the data.

## Limitations and errors

These pattern features aren't available. A constant pattern that uses one fails when the query is planned:

| Feature | Error | Instead |
| --- | --- | --- |
| Backreferences: `(ab)\1` | `invalid escape sequence: \\1` | Extract the group and compare it in a second step |
| Lookahead and lookbehind: `(?=`, `(?!`, `(?&lt;=`, `(?&lt;!` | `invalid perl operator: (?=` (and so on) | Combine two tests with `AND` or `AND NOT`, or check in client code |
| Named groups as `(?&lt;name&gt;...)` | `invalid perl operator: (?&lt;` | `(?P&lt;name&gt;...)` |
| Possessive repetition `a++` and atomic groups `(?&gt;...)` | `bad repetition operator: ++`, `invalid perl operator: (?&gt;` | `a+` |
| Free-spacing mode `(?x)` and option `x` | `invalid perl operator: (?x`, `Unrecognized Regex option x` | Write the pattern without spaces |
| Repetition over 1000: `a{1001}` | `invalid repetition size: {1001}` | Split the pattern, or test the length with [length](/develop/docs/dev-database-reference-sql-functions-string-length) |

Other pattern errors name the problem: `missing ): (`, `unexpected ):`, `missing ]: [`, `no argument for repetition operator: *`, `trailing \`, `invalid escape sequence: \\k`.

Nested repetition is safe to use: `(a*)*c` on a string of 50,001 characters returns `false` without error.

```
run("SELECT regexp_matches('abab', '(ab)\\1')")
run("SELECT regexp_matches('ab', 'a(?=b)')")
run("SELECT regexp_matches('ab', '(?<x>a)b'), regexp_matches('ab', '(?P<x>a)b')")
run("SELECT regexp_matches('abc', 'b', 'x')")
run("SELECT regexp_matches('abc', '(')")
run("SELECT regexp_matches(repeat('a', 50000) || 'b', '(a*)*c')")
```

```
InternalError: Invalid Input Error: invalid escape sequence: \\1
InternalError: Invalid Input Error: invalid perl operator: (?=
InternalError: Invalid Input Error: invalid perl operator: (?<
InternalError: Invalid Input Error: Unrecognized Regex option x
InternalError: Invalid Input Error: missing ): (
[(False,)]
```

## Similar functions

The other pattern languages, and how each matches one character in `&apos;abc&apos;`:

| Function | Pattern language | Matches | Case |
| --- | --- | --- | --- |
| [LIKE](/develop/docs/dev-database-reference-sql-functions-string-like) `&apos;a_c&apos;` | `%` and `_` | the whole string | sensitive |
| [GLOB](/develop/docs/dev-database-reference-sql-functions-string-glob) `&apos;a?c&apos;` | `*`, `?` and `[...]` | the whole string | sensitive |
| [SIMILAR TO](/develop/docs/dev-database-reference-sql-functions-string-similar-to) `&apos;a.c&apos;` | regular expression | the whole string | sensitive |
| [regexp_full_match](/develop/docs/dev-database-reference-sql-functions-string-regexp-full-match) `&apos;a.c&apos;` | regular expression | the whole string | option `i` |
| [regexp_matches](/develop/docs/dev-database-reference-sql-functions-string-regexp-matches) `&apos;^a.c&#x24;&apos;` | regular expression | anywhere, unless anchored | option `i` |
| [regexp_escape](/develop/docs/dev-database-reference-sql-functions-string-regexp-escape) | turns text into a pattern that matches it literally |  |  |

```
run("SELECT 'abc' LIKE 'a_c', 'abc' GLOB 'a?c', 'abc' SIMILAR TO 'a.c', regexp_full_match('abc', 'a.c'), "
    "regexp_matches('abc', '^a.c$')")
run("SELECT regexp_matches('a.b', 'a.b', 'l'), regexp_matches('axb', 'a.b', 'l'), "
    "regexp_matches('axb', regexp_escape('a.b'))")
```

```
[(True, True, True, True, True)]
[(True, False, False)]
```

Clean up:

```
run(f"DROP TABLE IF EXISTS {TABLE}")
run(f"DROP SCHEMA IF EXISTS {SCHEMA}")
conn.close()
```

```
[]
[]
```

## See also

- [Regular expressions](/develop/docs/dev-database-reference-sql-functions-string-regular-expressions): all the regular-expression functions.
- [Search and split](/develop/docs/dev-database-reference-sql-functions-string-search-and-split): plain-text search and the `LIKE` family.
- [Where filters run](/develop/docs/dev-database-reference-sql-select#where-filters-run) and [pushdowns](/develop/docs/dev-database-reference-pushdowns).